An optical motion sensor is provided for monitoring movement of a mechanical device, particularly such as an infusion pump of the type used for controlled delivery of medication to a patient. The motion sensor includes a disk member mounted upon a selected mechanical output component driven by the infusion pump, such that the disk member undergoes displacement during normal pump operation. The disk member defines a pattern of reflective and substantially nonreflective surface zones which are alternately illuminated by a light source during normal pump operation. A light detector responds to the level of reflection from the disk member to provide an output signal which varies according to disk member displacement. This output signal is connected to a controller for comparison with drive signals supplied to the infusion pump, to verify proper mechanical output operation of the pump.