How can I detect forfeited bills through the use of watermark?

Hold the bill up to the light, on the left hand side it will have a line running through it kind of like a line around the top of a pack of cigarettes that you pull on to open the pack. If the line is there it is a legal bill. While still holding it up to the light there should also be another picture of a president in the lower right hand corner (you won't see this if holding it down.) This is true on the newer bills (not sure about the older ones)But you can also buy the counterfeit pens that everyone else has mentioned. You just put a little mark on the bill with it and if it stays light colored the bill is legal if it turns dark it is probably counterfeit. I run a business and the employees must check all the bills they take in with one of these pens. They work very well.