Check the security thread: it should be embedded and visible when held to light
Check the watermark: a faint portrait should appear when held to light
Check the color-shifting ink: the number 100 on the lower right should change color when tilted
Check the 3D security ribbon: blue ribbon with bells and 100s should move when the bill is tilted
Check the raised printing: genuine bills have a textured feel on the portrait and numbers
Check the paper: authentic bills are made from a cotton-linen blend, not regular paper
Check the microprinting: tiny text should be sharp and readable under magnification
Check the serial numbers: they should be evenly spaced, sharp, and match each other
Check the Federal Reserve and Treasury seals: they should be crisp and properly aligned
Check the portrait and borders: details should be clear, not blurry or faded
Compare it to a known real bill: look for differences in size, color, and print quality
Use a counterfeit detection pen only as a quick first check, not as proof
Verify with a bill scanner or UV light if available
If several features look wrong, treat it as counterfeit and do not accept it