Krebs on Security:

Once you understand how easy and common it is for thieves to attach “skimming” devices to ATMs and other machines that accept debit and credit cards, it’s difficult not to closely inspect and even tug on the machines before using them. Several readers who are in the habit of doing just that recently shared images of skimmers they discovered after gently pulling on various parts of a cash machine they were about to use.

I’ve gotten into the habit of doing this at ATMs I go to. Luckily, I haven’t found any skimmers but better to be safe than sorry. I also rarely have any money so skimmers won’t get much out of me regardless.