For an approximation, do the following:
- Buy a voltmeter from Radio Shack or other electronic stores.
- Connect the voltmeter to the speakers leads.
- Start the receiver/amplifier.
- Play a test tone (e.g., 1kHz).
- Turn up the volume and note the volts on the voltmeter.
- Square the volts and divide it by the resistance and that will give you power.
Power = V^2 / R
A better way is to connect big resistors to all the channels and use an oscilloscope. Details are in this
link.