A test procedure and a metal detector having transmitting coils, receiving coils and an electronic processing circuit adapted to detect variations in the signals received by the receiving coils against a reference value. The metal detector, also includes a test module which has a selector able to detect a test request, and a controller, used when a test request is detected by the selector, able to compare the signals from the receiving coils during the subsequent passing of a known standard reference object with a predetermined response through the detector.