How can you tell a real 100 bill from a counterfeit one?

A current real US $100 bill has :

• A plastic security thread running though it
• A watermark of Benjamin Franklin
• Color-changing ink on the lower left of the front

Older ones don't have significant security features. About all you can look for would be tiny red and blue fibers that were embedded in the paper. Also check the seal to see if the points are sharp and evenly spaced.

The novice can test for fake bills using a chemical pen. However, the pen only tests the linen, so the bill may be a smaller denomination that has been altered.