The watermark is the shadow of the portrait that appears when you hold the bill up to light. "That is one of the easiest ways for the common citizen to identify counterfeit versus genuine," DeSantis said. Periodically, there are people who attempt to recreate the watermark, he added, but it tends to be of very poor quality.
Counterfeiters whose bills do have watermarks are usually printing large-denomination bills on paper from small-denomination bills that they have bleached. People at stores usually only care that there is a watermark within the bill, Kersten said, but the watermark portrait must actually match the printed portrait to be genuine.
Pictured above is a forged $100 bill with a picture-perfect image of Benjamin Franklin -- but a watermark of Abraham Lincoln.