document updated 14 years ago, on Feb 28, 2010

We haven't had a problem with counterfeit bills yet, but we handle enough cash to need to be aware of it.

Features to check for, to ensure a bill is genuine:

the paper itself — the paper used for US bills feels really different
plastic strip [introduced in $100 in 1991, $20 in 1990, $10 in 1990, $5 in 1993]
- hold the bill up the light, and you should see a strip that reads "USA TWENTY", "USA FIVE", etc
- hold the bill over a UV light, and the strip fluoresces a specific color (see here for color and location)
color shifting ink — the number in the bottom-right corner changes color when viewed from different angles
portrait watermark — when held up to the light, there's a second portrait that (MAKE SURE the two portraits MATCH)
raised ink on collar — the ink printing process used means that all ink on the bill is raised, it's thicker than normal ink. The collar area of each portrait has a pattern that makes this especially clear. Run your fingernail over the collar, and make sure you feel the ink ridges.
red and blue fibers — these should be embedded throughout the paper, not just printed on top (yeah, okay, every webpage says that, but how the heck do you actually tell the difference?)
iodine marker — some stores use this, but it's a VERY basic check, designed only to separate printer paper from treasury paper. (note that semi-sophisticated counterfeiters bleach $1 bills and use the paper to make $100 dollar bills... the marker is only designed to catch very casual (random-Photoshop-at-home) counterfeiters)