I recently saw a counterfeit $100 bill. It was made from a $5 bill. They had bleached a $5 and then printed a $100 on the “blank” paper with a color lazer printer. The pens retailers use to test the paper did not detect it. The only way to tell was to look at the strip in the paper and you could clearly see it was a five spot.
The face of Lincoln might give it away too.