Theoretically, a solution would be:
- Record the exact same signal twice, one time stereo and one time mono.
- Pick a mixing plugin of your choice (e. g. this free https://www.maat.digital/2buscontrol/)
- Mixdown/export the stereo recording as mono.
- Compare the mono export to the mono recording, e. g. by phase-inverting the export's signal and summing both.
In case of two identical recordings this should result in a zero-line signal. Of course, you will never have two absolutely identical recordings. There will be differences. But you can come close:
- For this test, put your device on a fixed and stable stand.
- Record a simple signal from another electric device, like maybe a tone from a speaker.
- Use high volume to increase the Signal-to-environmental-Noise-Ratio.
- Take care of low environmental noise, of course.
The standard comparing procedure with phase invertion (described under 4.) would probably not give a clear result in this case. But comparing both signals by ear should at least give you the idea, if left and right are levelled out equally enough for your recording purposes.
After all, you might probably cross-check using a third recording of the same signal, if you manage to flip the device perfectly by 180°, swapping left and right channel thereby. Of course, the same limitations apply again. Plus some physical differences of the two mic capsules.
So after all, this remains to be an approach, but maybe the best one I can think off.