That's exactly why the $5 note underwent the colonizing process, and along the way the watermark of Lincoln was replaced by watermarks that say "5" and "FIVE" several times. Actually, the watermark of Lincoln was a much better match for the watermark of Grant in the $50 note, but once the fraudsters got the notes cleaned off, turning them into $100s was more profitable.

One day I watched a cash register clerk holding a note up to the overhead lights in a Dollar Store in Federal Way, WA. I asked if she was checking for the watermark. She replied no, she was checking to see that the watermark matched the portrait. Pretty savvy - the only person I've ever encountered who did that. They usually just swipe that ubiquitous yellow marker across the note. (That same store would do that to my $2 bills, every time.)

On another occasion I was paying my bill at the Denny's down the road from the Crowne Plaza in Rosemont, and suddenly the clerk was bathed in intense blue light from the waist down. She had a very powerful UV light under the counter, and was checking the reactions of the polymer strips in the notes that I had handed her. That's the only time I have seen that behavior, also.

At least some clerks are getting training in security features other than that d....d yellow pen.