Microwaves have 2 sides of power, low & high. Low side line voltage & below before the magnetron. High side voltage are components associated with magnetron ie. capacitor & magnatron. I isolate the systems to do my troubleshooting.
I check the schematic & start at the control board. Do I have 120v coming into board? If I do that pretty much eliminates all fuses & thermostats before board, Do I have 120v coming out of board, if I don't pretty much says board is bad. If 120v coming out I start checking high side for problems.
I am by no means an expert on M/W's & I only do calls for high dollar built in matching sets of Microwave & ovens. Those are few & far between & they do eat up a lot of my time. Not real money makers, I do it mostly for the PR, gets me referrals for lucrative jobs.