In the past when I worked at a retail store occasionally fake money would get passed off every now and again. If we sent fake currency in the bank deposit, black car tinted window feds would sometimes get involved. Specially if a lot of fake currency was getting passed off in the area.

The pen markers are near worthless for detecting fake money. If you have handled lots of american cash, you can typically tell right away if the paper is wrong by the way feels in your hand. As stated above the feel of the money, watermark, and strip are the best ways to tell.