Read these articles if you know whats good for you.

A good write up showing how various skimmers can be identified:
http://www.lowtechcombat.com/2010/02/atm-card-skimmers-exposed.html

http://www.koco.com/news/30691761/detail.html

http://www.protectidentity.com/no-surprises-credit-card-skimming-prevention/