Amber mark passes the test, while a dark brown or black mark means the bill is suspect.
Highly effective and inexpensive tool for detecting bad bills. Simply make a small mark on U.S. currency. Chemically sensitive ink works instantly. An amber mark passes the test, while a dark brown or black mark fails, meaning the bill is likely counterfeit. Works on all new design U.S. currency (bills series 1959 and later).