A plug released into an oil or gas well carries one or more metallic strips that affect the inductance of a resistive-inductive bridge circuit mounted adjacent a conduit through which the plug and fluids are pumped into the well. This unbalances the bridge circuit to produce a signal preferably uniquely identified with the plug so that an indicator can be activated to confirm proper release of the plug into the well. When an amorphous metal is used in the metallic strips, the unbalancing of the bridge produces a signal having an identifiable harmonic of a fundamental frequency of an oscillator signal driving the bridge circuit, which harmonic is not present in other signals resulting from the bridge circuit being unbalanced by other materials flowing in the conduit.