Hi arj you could use Ohm's law. do it with a simple multimeter. connect multimeter in parallel to your speakers. select AC voltage - lowest setting. play music at the level u normally listen to. measure the highest reading you get. Square it and divide it by the nominal resistance (or impedance if u like it that way), usually, 4 or 6 or 8 (measure this initially with meter in resistance and speaker terminals removed). u get the power consumption for that channel.
this is a ball park method of evaluating but gives you a fairly good picture of ur amps output (in watts)