That's not always true lol, especially with speakers, look at Infinity Kappas, 350RMS 1400MAX. That seems like a problem in my opinion, and also that those will never go above 350RMS. That's not good lol. It's that 1400W MAX that sells them.
peak ratings are no clear indication of anything. the only way to really get the amplifier's output is either to look it up with the maker of the amp, or measure it using a non inductive dummy load and a scope.
Glass, do you REALLY think the company is going to tell you the truth? A simpler way instead of a scope would be a high input power meter of sorts. I use one on my stereo that does 200W of max power. Plenty for me, but it's quite TEH usefulness.