How do I find the actual wattage an amplifier is putting out?

It is actually quite tricky to measure this accurately.
The reason is that the power depends on the impedance of the load (i.e. the speaker) and the latter can be VERY frequency dependent for a real speaker.

Another problem is that the "maximum power" is not really that interesting, the distortion increases with power and at some point the amplifier will start clipping, this is not really big problem initially (at least not if the amplifier is clipping "gently") but sooner or later you will reach a point where the music sounds terrible and -worse- you risk destroying your speakers (an amplifier that is clipping outputs DC that heats up the voice coil).

Hence, measuring power of an amp feeding a speaker in "real time" is therefore somewhat tricky; the only way to do it is to measure both the voltage and the current from the amp but that is NOT something I would recommend unless you know what you are doing (voltage is not the problem, but measuring the current is).
The result will also depend on how long you are averaging (i.e.. the "time window") since peak power can be MUCH higher than the median power for real music.

Anyway, the standardized test is to measure the voltage across a 8 Ohm power resistor at 1% distortion and then calculate the power from P=V^2/R. This is what the wattage rating of an amp means.

REf : f95toli