A method is disclosed use with a circuit device that includes a circuit having a predetermined voltage-current characteristic and a detector configured to detect a voltage-current relation of the circuit. The method includes using the detector to detect the voltage-current relation of the circuit, and indicating if the detected voltage-current relation differs from the predetermined voltage-current characteristic. A circuit device includes a circuit having a predetermined voltage-current characteristic, and a detector configured to detect a voltage-current relation of the circuit. The circuit device is configured to indicate if the detected voltage-current relation differs from the predetermined voltage-current characteristic.