How can i calculate how many watts my amp is actually putting out? Just to verify what the manufacturer is saying on the box. IE to be able to tell if your amp which is rated at 600w rms is actually putting out about 600w rms.

I heard something about a volt meter and a couple of equations?

If you look at a lot of youtube videos about a person's subwoofers the person who created the video will say ive got this amp and its doing 1200w rms and then you'll look down at the comments and some basshead will be talking about how they seriously doubt that amp has that much output and that the rms is actually much lower than that... so that makes me curious of how to calculate what mine is doing

Thanks!