Read this article and it tells you exactly how to measure output power with an AC volt meter. All of my tests were at 40 hz.
What this does not tell you is the distortion level. I do know from a YouTube video that a guy made where he tested the amp I have on an amplifier dyno is the amp is capable of roughly 600 watts continuous at less than 1% THD.
So, by setting the gain so the amp is putting out about 500 watts then I know the amp is not clipping. My maximum volume on the stock radio is 34. This is approximately 75% of max volume which typically is the highest you want to turn the volume up. It is more or less an industry setting if you have no means of putting an O scope on the speaker level outputs on the stock radio.
I’m not an electrical engineer but I have been around the block a few times.