I thought I'd pop in since there were some interesting suggestions here, although not all were very accurate.
You cannot simply measure the output at the speaker terminals unloaded. This will get you close to the rail voltage, but not any indication of true power rating.
The very minimum you will need to measure unclipped power is as follows:
1) 8 ohm non inductive power resistor
2) Oscope
3) Signal Generator
Adjust the signal generator to around 1Vrms and set the frequency to 1kHz. Attach the resistor to the speaker terminal under test. Then increase the master volume level until you start seeing clipping on the oscope. Turn it down slightly until you no longer see clipping.
Take the RMS Voltage reading of this waveform and turn off the source.
Do simple math: V^2 / R
Repeat at 1/3 octave intervals or at least 20Hz and 20kHz.
***It is recommended to monitor the line voltage with an RMS meter and make sure it doesnt sag. It should stay within +-10Vrms of 120V nominal.
For more ellaborate tests given this test equipment, see our article:
Basic Amplifier Measurement Techniques