(TNS) — When filling your tank, be aware of shady gas pump skimmers that could leave a huge dent in your bank account.

What is a skimmer?

It’s something commonly used to commit fraud. They are small devices thieves place on or above the card readers at gas pumps, ATMs and self-checkouts. The danger lies in the device copying or stealing your credit card information when you swipe your card, “skimming” the data.

Not only is the scam done in cities; it’s done in rural areas as well.

“A credit card skimmer is a device that transfers data from your credit card’s stripe,” Chris Hauk, consumer privacy champion at online privacy and security site Pixel Privacy, said to Reader’s Digest. “Skimmers are usually found on gas pumps or other point-of-sale devices, in areas that aren’t being monitored every minute of the day, as this allows the bad guys time to install a skimmer on a pump without being observed.”

Due to the widespread scam, it’s something everyone should remain vigilant about.

“Whenever a customer pays with their card, their card is read by both the skimmer and the original reader,” Paul Bischoff, a privacy advocate with cybersecurity website Comparitech, said to Reader’s Digest.

Once customers see their payment successfully go through, they seem to have no reason to be alarmed.

“Skimmers are designed to be discreet so that the victim doesn’t notice them,” Bischoff said. “The skimmer stores scanned credit card info until the person who put it there retrieves it.”

And from there, the next step is for the thieves to use your credit card for online purchases, racking up on unauthorized sales.

One way to avoid the scammers is to pay inside when needing to fill up, Reader’s Digest reported.

“Use a chip reader on the pump, if it is available,” Hauk said to Reader’s Digest, “and always use a pump that is in a visible part of the gas station, such as the pump right in front of the cashier’s window. High-visibility pumps are tougher to attach skimmers to.”

Curious to know what a skimmer looks like?

If a credit card reader is bulky, plastic, sticks out further than the machine, and has arrows that don’t line up, plus maybe a shake to it, it would probably be best to use another machine.

“Because they are attached to existing card readers, they tend to bulge out a bit,” Bischoff said to Reader’s Digest. “If I’m suspicious, I’ll often give the card reader a tug and prod to make sure.”

Here’s what else to do: