A photodiode and a phototransistor are positioned on opposite sides of a document subjected to verification for authenticity under transmitted light. A logic circuit determines the presence or absence of the security feature and correspondingly provides visual or audible indication thereof. The photodiode, phototransistor and related circuitry are arranged within an enclosure that is attached to a currency receiving device such as a cash register. The visual or audible indicators are mounted on the cash register for immediate indication of the currency verification to the cashier. A simplified capacitive proximity sensor can be employed, per se, for inexpensive applications or for redundant verification in combination with an optical detector. The shape of the capacitive sensor is configured to that of the security thread to discriminate between the security thread and the currency paper. Additional optical circuits are employed to detect counterfeit markings on the currency surface.