Basically all you can test is that the inverter is getting 120 volts to the gray & red wires when heating. Then you can check amp draw while it is heating, should be around 11 amps. My experience though with these tells me every time replace the magnetron AND inverter board together. The fact that you get a pulsing buzz along with the F7 indicates one of those components is bad. Too many times I have replaced just the inverter board, only to have the new inverter board ruined by a bad magnetron that otherwise tested "good". If you were going to try to replace only one part at a time, I would replace the magnetron first, as a bad inverter is not likely to damage a new magnetron. However, if you replace the magnetron and it works, don't expect the inverter to last long anyway as they usually don't after a bad magnetron has stressed them.