How can you tell if a US 500 dollar bill is real?

The last US $500 bills were printed in 1945, long before any American paper money had high-tech anticounterfeiting features like watermarks or fluorescent strips. Bills from that era did have a few features that can be used, though:

• There are tiny blue and red fibers embedded in the paper. You should be able to see them if you hold the bill up to a light.
• The paper should have a heavy feel very much like that of modern $1 bills that use the same quality paper.
• The "teeth" around the Treasury seal should be evenly spaced and sharply pointed if you look at them under a magnifying glass.

If none of these are conclusive, you could call a local currency dealer or appraiser and see if they'd be willing to examine the bill in person.