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.