Simply make a small mark on U.S. currency.
Chemically sensitive ink works instantly.
Amber mark passes the test, while a dark brown or black mark means the bill
COMPTOIR OIL-GEL INK COUNTER PEN. With its professional look, this medium 0.7mm tip pen is designed to be placed on desks and counters. The base attaches to pen with a coil and rotates 180 degrees...