If the marking shows as a bright yellow/amber the note is valid!
If the marking goes dark and stays dark the note is forged! D-TEK Counterfeit Detector Pen, 12/Box D-TEK Counterfeit Pens allow you detect counterfeit currency in seconds.Product highlights include highly effective, quick and simple, work on International and US Currency, and antimicrobial protection. Simply mark the note in question, for example, with a cross in an area that is lightly colored and not printed on. This product is PROP 65 compliant.