Get an amp probe and put it in line and run your experiments is my best suggestion. You'll need a watt meter in the output or an RF amp meter if you know what the impedance of the line will be.
Now if you get one of these meters, you can measure amps all the time. It comes with the current transformer.
http://www.go2marine.com/product.do?no=83298FWhat your looking for is throughput. power in vs power out equals efficiency of just the final stage.