Even without a PIN, duplicated Visa or MasterCard debit cards can be used to make online purchases.
Check lights, wiggle card slot
Most ATMs have a flashing or steady light at the card slot. If you don't see that, it could indicate that a skimmer's been attached. (But keep in mind that some older ATMs don't have those lights.)
Another precaution is to wiggle the card slot before inserting your card. If it's not securely attached or has a different color from the rest of the ATM, use another machine. (And always cover the keypad as you enter your PIN, because a spy camera may be watching.)
Even if you're unaware of problems when you use an ATM, you should always carefully review your bank statements as soon as you get them to detect any fraudulent withdrawals, as skimming scams often go undetected until accounts are drained.
Banks usually reimburse customers of ATM scams, but policies vary and some banks might not pay up if you wait too long.
Sid Kirchheimer is the author of Scam-Proof Your Life, published by AARP Books/Sterling.