Whether you're a newbie or a yoga veteran, it's not always easy to tell if your instructor is the real deal — or a (downward facing) dog.

1. "Hey, I just started doing yoga myself!"
Yoga Is booming: For proof, you needn't look any further than your health club, your cable-TV schedule — or even Target or Walgreens, where yoga mats and DVDs now share shelf space with household sundries. According to a Harris Poll last year, some 15 million Americans are practicing yoga — an increase of nearly 30 percent over 2002.

The problem is that there's no real standard for how much teacher training is required, so almost anyone can call himself an instructor. There are plenty of certification programs around, but while some offer thorough training — the Advanced Studies Program at the Yoga Room in Berkeley, Calif., requires 500 hours of classwork covering philosophy and anatomy — others are little more than weekend workshops.

How, then, to avoid the bad apples? In 1999 a group of instructors founded the Yoga Alliance, a consortium of yoga schools based in Reading, Pa.; affiliated schools must require a minimum of 200 hours of teacher training. Although joining the group is voluntary and many perfectly good teachers haven't signed up for its instructor registry, you can check to see if your teacher has at least attended a YA-approved program.

2. "Sure, we have mats you can borrow....How about a case of athlete's foot, too?"
You've probably seen yoga diehards heading to class, their telltale yoga mat bags slung over one shoulder. Yet many studios lend or rent the mats to their students, often charging a nominal fee of $1 to $2. Is lugging around your own mat really necessary?

Absolutely, according to Tim Cowen, a yoga instructor in San Francisco. "The one thing I'd tell new yoga students is, always bring your own mat — never use the ones at the studio," he says. Even though some studios do try to wash or disinfect their mats regularly, he says, "I can honestly tell you that most mats don't get sprayed on both sides." And just contemplate Bikram yoga, a popular yoga discipline that's performed in 105-degree classrooms so that muscles and ligaments relax as much as possible and the body can sweat out toxins. "Sure, it's supposed to be a dry heat in those classes," says Cowen, "but with 30 people sweating for 90 minutes? The room's a petri dish." Spend the $20 to buy your own mat — or even go without. The Yoga Center of Palo Alto, Calif., uses mats only for seated poses, headstands, shoulder stands and some backbends, doing all standing poses on wooden floors that get mopped before each class.

3. "You're not ready for this class..."
A few years ago Kim Rutenberg and her husband started attending classes at Jivamukti Yoga Center, a Manhattan studio popular with celebrities. They started out in a beginner's class, and after a month decided to try a more advanced class. They quickly found they were underprepared. "You go immediately into headstand and some other really tough poses, and we couldn't do a lot of them," says Rutenberg, a marketing executive. They didn't get hurt, but no one warned them about the degree of difficulty, she says. (The cofounders of Jivamukti insist the studio recommends beginning students complete at least three months of its basics program.)

Yoga classes tend to be rated by level of expertise — typically beginner, intermediate and advanced — but if you say you're ready for an advanced class, chances are no one at the sign-in desk will question you. The best thing to do, says San Francisco instructor Rickie Milmoe, is "call the studio ahead of time and say, 'here's how much yoga I've done,' and ask them which class is most appropriate. Don't try to guess." And be honest about your abilities. After all, you won't learn much if you get in over your head — and you might become too discouraged to continue.

4. "...and you could really hurt yourself."
Some yoga poses are universally acknowledged to be risky — in particular, inversions like shoulder stands and headstands. Since they cause blood to rush to the head and can raise blood pressure, these poses are particularly dangerous for anyone being treated for glaucoma or chronic headaches or anyone who's recently had a stroke; they're also risky for anyone who's more than 30 pounds overweight, since they compress the vertebrae in the neck. Good yoga instructors will caution a class before going into inversions and will keep a careful eye out for anyone doing the pose improperly.

But not every instructor takes such precautions. Software marketer Oonagh Kavanagh went to a studio in San Francisco that gave individual attention to each student doing inversions, but says when she switched to another studio, her new instructors didn't watch her at all. "It was just up-you-go," she says.

Often, though, it's your own enthusiasm and ego you need to watch out for. Donna Rich, a psychotherapist in Austin, Tex., recently pushed herself into a hamstring stretch she wasn't ready for and ended up tearing some connective tissue in her foot. "Wanting to show off is dangerous," she says.

5. "I'm just here to get lucky."
While many gyms, training schools and yoga teachers associations forbid liaisons between instructors and students, with all that bared skin and limber bodies, it's no surprise that some instructors break the rules. But matside flirtations can easily erupt into sexual scandals: Witness the 1994 meltdown at the Kripalu Center, a yoga retreat in Massachusetts's Berkshires led by the charismatic guru Amrit Desai. After Desai — who both preached and claimed to practice celibacy — admitted to having slept with students, he was ejected and the ashram's reputation tainted.

Serious stuff — but many yoga teachers still haven't learned such a lesson, making some studios as rife with drama as a nighttime soap opera. One New York City woman tells of a recent flirtation with her female yoga teacher that turned into an affair. She soon found out the instructor was simultaneously involved with two other women at the same studio — a student and another teacher. The woman ended the relationship and is now warning friends to avoid the teacher. If you suspect inappropriate attentions are being directed at you, you may not be imagining it. You have every right to protest or to find another class.

6. "Our health club offers yoga just because it's trendy."
Not surprisingly, gyms have jumped on the yoga bandwagon: As of 2002, 84 percent of all U.S. health clubs had yoga on their class schedule. But how seriously they take it is another matter. Edward Vilga, an instructor at Manhattan's Laughing Lotus studio, once taught at a popular health club. "One day, while my class was in savasana" — a stone-silent, meditative final rest period — "someone came in and started doing leg lifts in the corner." Vilga asked her to leave, but he says the club's management didn't back him up.

Although most gyms try to hire good teachers, they often don't provide a yoga-friendly environment. Many yoga studios discourage arrivals that are more than a minute or two late, but at some gyms, Vilga says, "students drop into a class like it's a cocktail party." Students who come in late aren't warmed up, he says, and often miss critical instructions.

But Karyn Taylor, a yoga student in Los Angeles, says that blocking out the clanging weights at her gym has become an essential part of her practice. "Yoga studios can be sort of surreal," she says. "Give me a real experience, and let me see if I can calm my mind within it." Try yoga both at your gym and at a studio, and see what feels right for you.

7. "You don't have to let me touch you."
If you're going to your first yoga class, you may be in for a bit of a surprise: Not only will the teacher give you verbal instructions, he or she will probably also adjust you manually.

What you should feel is a gentle correction or even just a calming touch. According to Yoga Alliance President Hansa Knox, adjustments should never force you into the perfect version of the pose; they should just help you discover what's right for your individual body. Still, some inexperienced teachers can be overzealous with their hands.

Worse than recklessness, though, is creepiness. Susan Delz, a store manager in Palm Beach Gardens, Fla., recently walked out of a class because she felt the substitute teacher was inadequate, but she later learned that other women had complained about him touching them inappropriately during adjustments. "Even if he hadn't meant to cross any boundaries, he did," she says.

A teacher should always forewarn you when he's going to touch you — and should never touch you inappropriately. And if you don't want to be touched at all, it's fine to say so. "You're always the boss," says Knox.

8. "Yoga isn't a cure-all."
Several recent mainstream medical studies have shown that yoga can help alleviate everything from chronic back pain to asthma. One study even found that people with coronary heart disease who do yoga have fewer angina episodes. But yoga doesn't have all-healing powers, as some high-profile yoga gurus have implied.

Bikram Choudhury, for instance, displays prominently on his Web site testimonials from students who claim his brand of yoga has helped alleviate symptoms of everything from rheumatoid arthritis to hepatitis C to anorexia and bulimia.

While yogic tradition has long held that certain poses relieve certain ailments — seated forward bend for sinusitis, or headstand for insomnia, for example — not all of these claims are supported by hard evidence. "What we do say is that yoga's primary effect is to bring all the body's systems into balance," says Trisha Lamb, associate director of the International Association of Yoga Therapists in Prescott, Ariz. "Someone with cancer may be helped by yoga, but you should not assume yoga will cure cancer." Nor will it make you look like Christy Turlington. Exercise of any kind can increase muscle tone and help you lose weight, but there are some things only good genes — or plastic surgery — can give you.

9. "I teach only one kind of yoga, and guess what? It's not the one for you."
There are dozens of different types of yoga, ranging from Integral classes, which include lots of chanting and prayer and minimal physical activity, to fast-paced "flow" classes to Iyengar yoga, which emphasizes the alignment of the body and uses props like straps and wood blocks to help students into positions, to hot and sweaty Bikram yoga, which consists of 26 poses done in a room heated to 105 degrees.

Which kind of yoga is right for you? It depends on what you're looking for — relaxation, say, or a rigorous physical workout. Trying out different classes is the best way to determine the type of yoga that best suits your goals and abilities. Even more important than what kind of yoga you choose, though, is how much the teacher inspires you. Yoga instructors have dramatically different styles — some are as authoritative as gym coaches, while others are more touchy-feely. One class with a teacher who lights incense and chants about world peace, for example, might turn you off to the practice altogether. "Different people are drawn to different philosophies of teaching," says Larry Hatlett, director of the Yoga Center of Palo Alto. The best thing to do is try out a few and listen to your instincts, says Nina Zolotow, a writer and teacher-trainee in Berkeley. "If you look forward to seeing your teacher and to going to class, you can't lose."

10. "You don't really need to come to the studio every day."
Attending yoga class regularly will keep you motivated and inspired — and regular instruction will help you learn the proper way to hold yoga poses, especially if you are new to yoga. But one of the biggest benefits of yoga is its portability: As long as you have a flat surface and enough room to move, you can do it almost anywhere.

Indeed, the best instructors will encourage you to establish what's called a home practice — and with fees approaching $20 per class at some yoga studios, you can save a bundle. In his recent book, Moving Toward Balance, yoga expert Rodney Yee lays out an eight-week program carefully designed to keep you working happily at home. "For most people, the biggest stumbling block to doing yoga at home is the fear that you're not doing it right," says Zolotow, the Berkeley writer. For the best results, she says, yoga students seeking to practice at home should just "listen to your body and use your common sense about what you're capable of." And try not to worry about whether you're doing it exactly right. "There's really no way of doing it wrong," says Zolotow. "As long as you do it."