How to identify fake US$ currency notes?

Question

If somebody gives me a US$100 bill, how can I tell if the bill is fake?

Answer 1

I will stick here with things that weren't mentioned above: If you handle a lot of $100 bills, buy a small UV (black) light. The mylar thread glows when exposed to UV light and each denomination glows a different color.
One common counterfeiting practice is taking a $5 bill, bleaching the ink off, and using a color copier to print the $100 design on it. The tipoffs are that the watermark won't match (Lincoln vice Franklin), and the mylar thread won't glow the right color or have the correct wording (FIVE vice 100). Since the paper is correct, the pen is no help on these.
US $100 bills dated before 1990 don't have a mylar thread. I am now reluctant to accept them.

Answer 2

There are numerous ways to check. You can buy a currency marker (a pen that marks the bill a ceretain color if it is real vs. fake) used by banks, casinos etc. and cost about a dollar at any office supply store. Also on a $100 bill, look for red and blue tiny fibers in the paper, a watermark of the presidents face just to the right of the printed face (hold it up to a light), and turn the bill sideways and look for an embedded line about 1 1/2 inches down that has the numbers 100 (and maybe some words that say one hundred). Those are the simple ways.