how to test amplifier real watts?

Can someone tell me how to test an amplifier's real wattage output (before clipping) to the speaker using a Multimeter or Amperage Clamp Meter? I have a signal generator cd which has various frequencies recorded on it - so which would be the best test frequency? Thanks!

Here's a good article.  http://www.bcae1.com/measpwr.htmSupport the12volt.com

I'd suggest a true-RMS power meter and your speaker. Maybe a true-RMS ammeter will suffice assuming the rail voltages remain nearly constant. Take ammeter readings when unacceptable becomes acceptable distortion (try various sound profiles) and that is your system's max RMS output. And both the above include the resistive heat lost to the water etc. (And don't mix water with electricity! It tastes tingly - but kids - don't try it - unless under approved parental supervision.)