I have a Kill-A-Watt meter (you can find them off Amazon), and it measures the AC power draw off the wall socket. Not the same thing, but you know the amp is providing no more than the AC draw. In general class A/B amps are around 50% efficient, so if you draw 100W you are probably getting around 50W amp power.

These devices are cheap and useful to play with. I'm using it with my computer stuff. My computer in "shut down" position is 10W while in "standby" is 12W, so I know that the computer still draws power when "shut down." However, in standby, using only 2W more, I can restart in about 2 seconds. Useful thing to have learned.