As an electronics geek it took me a while to understand how GA headsets are wired. I can share how I'd bench test them with the rudimentary equipment I have, if that helps. Out of curiosity I just tried with one of my headsets and this method works with mine. But my understanding of GA headset wiring may well be incomplete. Perhaps different headsets do more wildly varying things than what I know about.
I connected a 330 ohm resistor in series through the headset microphone connector (thinner plug, ground furthest from the tip, and signal in the middle ring) with 10V from my bench power supply. Measuring the potential across the microphone connection with an oscilloscope then gave me around 5V with around a +/- 1V peak amplitude when speaking at the level I would in the aircraft. This is with ANR on - with ANR off I suspect the result will have been similar but different.
As far as I understand GA headset microphones "pretend" to be graphite microphones with some circuitry even though nowadays they are all electret - so should modulate resistance - and historically aircraft would just shove through the regulated 24V-ish supply. One GA headset manual I have (for my headset) specifies a matching impedance of 150-1000 ohms and 8-32V; hence my selection of 330 ohms and 10V.
If you have a multimeter you could try to shove through perhaps 18V by using two 9V batteries, a suitable resistor and the headset microphone connector all connected in series and see what potential you can measure across the connector. That'll at least tell you if you have an open circuit. But since I'm not confident I fully understand GA headset circuitry, please don't blame me if you blow something up