Attaching a scope to the Zone 2 output would be the best diagnostic tool. Protection circuits are usually designed to detect over current and DC offset transgressions, but they can sometimes detect clipping (voltage limit). A DC voltmeter can be used to check for simple DC offset, but a simultaneous AC signal will probably frustrate the voltmeter. Due to typical voltmeter frequency response and waveform limits, it will be of marginal use to prove or disprove clipping. One indication would be that the voltmeter reading stops increasing as the amplifier input voltage is increased, but using the amplifier's Volume control to set the amplifier input voltage will be difficult to deal with because the control taper is not likely to be linear.

I would be tempted to build or buy a gadget similar to this.