Measure power output without oscilloscope

I don't have an oscilloscope. Is it accurate to some degree to crank up the input level of a sine wave until the amplifier clip light comes on?
Power can be either peak or average. Relationship between them depends on the shape of the signal. For sinewaves average is a half of the peak power. Knowing peak power (peak voltage squared over speaker impedance) will let you calculate average power - a very useful thing if you listen to sinewaves.

touch the speaker wires to your tongue and measure how high you jump!

with the power output of some of the modern  amplifiers, that can actually be quite the alarming experience.

eg, a 1600 watts into 8 ohm class d power amplifier, can swing quite the alarming wave. (betwixt a pair of fingers). 

Feel that reggae bass line. literally. muscles just a' flappin' away to the beat....

Ask me how I know....

sorry guys, I was joking.   I remember reading that there was one amp company that demoed their amplifier being used as a power supply for an arc welder.