This does not fall under comptuers, but I thought it was very important.

Consumerist has a link to a PDF presentation of how to look for a scanner. http://consumerist.com/5221192/id-an-atm-skimmer Be sure to click on the link on that sit to the PDF. It is for a bank in Australia, but is still show you what to look for.

Here is a video talking more about skimmers.