How do I know the true wattage output of my receiver?

[Big Daddy]

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.