what you calculate on paper will be different then what you'll actually get from the amp. paper calculations don't take into consideration a lot of different factors.
Gain Setting Voltage = Sqrt(Watts RMS*Load)
reverse that and i'd say thats about the best way. use a DMM to measuse the AC voltage at the output terminals when the speakers are unhooked and volume is 3/4 or where ever you usually listen. leave Watts out of the equation and solve for watts.