One of the more successful tools of 21st century crooks is the skimmer. Thieves attach these sneaky gadgets to steal your credit or debit card’s magnetic strip information at ATMs, gas pumps and other places where you swipe your card. The thieves use this information to make a clone of your card, and then they use the clone to drain your bank account or run up huge bills and trash your credit.

That's one reason credit card companies and stores are switching to EMV cards, or “chip” cards, but it will take a while before every retailer supports them, which leaves you vulnerable.

How bad is skimming?

Using skimmers, thieves have stolen untold millions of dollars. Last year four men were arrested for allegedly using skimmers to steal $2.1 million at gas stations across the South. If the average gas transaction was $50, they had to skim 42,000 transactions to rake in $2.1 million. That's a widespread, long-term threat.

Skimmers are available for sale to anyone who wants to buy them, and they can show up anywhere. Just glancing at the news for the last few months shows that skimmers were found at dozens of gas stations in Detroit, St. Louis, Cincinnati, cities in Florida and at bank ATMs around the country. According to the New York Police Department, ATM skimmer use has tripled in Queens this year.

So how do you spot a skimmer before it snags your information? Here's what you need to look for.

Overlay skimmers

An overlay skimmer fits over the card reader slot of an ATM or gas pump and is usually modeled, or in some cases 3-D printed, to look like the part it's covering. But an old or low-quality one might not be the same quality or color as the rest of the machine. Or maybe it protrudes a bit too far or isn't installed straight. If it looks like it doesn't quite fit, or if it’s loose, that's a warning.

You can also look around for additions to the machine that might hide a camera that’s pointed at the keypad. This is often how crooks get your PIN. It might be installed on the ATM, or even on the wall above it. Hackers have hidden cameras in fake outlets, lights and other things that wouldn't immediately catch your attention.

It's fairly easy to defeat these types of skimmers. Simply cover your hand when you type in your PIN and the crooks won't have all the information they need to clone your card. Watch this video to see how effective this precaution is.

Advanced skimmers

Unfortunately, hackers quickly spotted the weaknesses of skimmers and came up with options that are more difficult – or even impossible – to detect.

A good example comes from Brazil. It's an overlay skimmer, but instead of overlaying the card reader, the entire front of the ATM is fake. See the unbelievable video.

The gas station thieves I told you about earlier hid their skimmers inside the pumps. They were even equipped with Bluetooth, so the thieves could drive by and extract the collected numbers and PINs wirelessly.

Another skimmer is thin enough that it fits right inside the ATM or gas pump's card reader slot. There's no external sign it's there, but it's busy swiping your info. This is what it looks like.

Unscrupulous employees of a restaurant or store may have handheld skimmers you'll never see. Or they might pull out POS terminals that are really skimmers in disguise. They'll even print out a receipt.

Not many crooks have these advanced skimmers yet, but they'll get less expensive and more widespread in a few years. Fortunately, banks and retailers are switching over to EMV cards that have a chip instead of a magnetic stripe. 

Mobile payment options are also becoming more popular, and they don't have the security flaws of traditional cards.  

In the meantime, you should try avoiding swiping your card in the places most likely to have skimmers. Here are the three most risky places to swipe your debit card.

You should also set up text alerts on your cards so you know when you or anyone else swipes them. You can tell right away if hackers have stolen your information. Learn more about text alerts.

On the Kim Komando Show, the nation's largest weekend radio talk show, Kim takes calls and dispenses advice on today's digital lifestyle, from smartphones and tablets to online privacy and data hacks. For her daily tips, free newsletters and more, visit her website at Komando.com. Kim also posts breaking tech news 24/7 at News.Komando.com.