There's a rash of counterfeiting going on in my area. This involves washing the ink off 5 dollar bills and re-printing them as higher denominations. This defeats the chemical pens that will reveal the use on substandard paper.
It is possible to hold a bill up to some light and check whether the watermark matches the portrait printed on the face of the bill. But this is time consuming and rarely done by clerks in a hurry.
There's one other method that works well, but I've only seen it in use once. If one illuminates US currency with a UV 'back light' the plastic security strip fluoresces. Each denomination's strip is in a different location and its strip fluoresces with a different color. A simple guide affixed under the lamp is used for comparison, although the clerk I talked to said that she got used to identifying the strip/color location very quickly. All one needs to do is drop their bill on the counter, next to the cash register and lamp. If its bad, a clerk can tell at a glance.
This seems like pretty inexpensive and simple technology. So why don't more people use it?