An overlay skimmer is one that fits over the card reader slot of an ATM or gas pump. For old or low-quality overlay skimmers, there are a few things you can look for.
It's usually modeled, or in some cases 3-D printed, to look like the part it's covering. However, it might not be the same quality or color as the rest of the machine. Maybe it's protruding a bit too far, or not installed straight. If it looks like it doesn't quite fit, then that's a possible warning.
You can also look around for additions to the machine that could hide a camera pointed at the keypad. This is often how crooks get your PIN. It might be installed on the ATM, or even above it.
That's the case with a skimmer a passenger recently discovered on a MetroCard Vending Machine in the New York subway. Here's the overlay part (after it was taken off the MVM):
And here is the camera installed above the machine disguised as a plug. See the pinhole on the bottom for the camera lens?
For these types of skimmers, it's actually fairly easy to defeat them. Simply cover your hand when you're typing in your PIN and the crooks won't have all the information they need to clone your card. Click here to see how effective this simple precaution is.