The security thread is one of the most distinctive security indicators of an authentic bill.
If you hold any bill up to the light, you will see a security strip on an authentic bill. It will have “USA” and the Bill’s denomination, aka “USA 20” running vertically on the bill.
The ultimate test, is to put the bill under a uv or black light and make sure the bill glows the correct color. This extra security measure was originally added to provide a quick way for bar owners (one of the businesses most commonly targeted to dispose of counterfeit bills) to check the legitimacy of a bill.
Here are the location and colors of the security threads on different bills:
$5 – Right side. Glows blue under UV light
$10 – Right side. Glows orange under UV light
$20 – Far left hand side. Glows green under UV light
$50 – Right around the middle. Glows yellow Under UV light
$100 – On left side. Glows pink under UV light