Some cheaters do more than just fake a signature. Sometimes they change numbers on a check or swap pages in a contract. A document examiner for attorneys uses special tools to catch these tricks.
They shine ultraviolet and infrared lights on the paper. These lights show if someone used two different pens. For example, if someone adds a zero to a check with a different pen, the special light will reveal the trick. They can even read invisible marks left on the paper from a pen pressing down on the page above it.
