Smart Money Counterfeit Bill Detector Pen For Use W-u.s. Currency, DozenHighly 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). Counterfeit Bill Detector Type: Pen; Marking Color: Amber; Counterfeit Indication Mark: Black; Dark Brown; Currency: U. S.