A detecting method of a detecting circuit with a plugged test lead. The method includes setting an operation mode of the detecting circuit by a recognizing unit, transmitting a signal to the recognizing unit by an oscillating unit when the test lead plugs or unplugs into a split contact jack, receiving the signal from the oscillating unit by the recognizing unit, and determining a condition of the detecting circuit, and sending a message to a user in accordance with the determining result of the recognizing unit. Further, the oscillating unit is stopped from oscillating and outputs a quiescent state voltage to the recognizing unit when the plugged test lead is inserted into the split contact jack.