Good morning Edwin Lebish! The HV capacitor holds 4000-6000 volts, even uplugged. Don't attempt to check voltage on the HV circuit. It's much safer to check the amperage with a line splitter. This amperage test is quick, easy, accurate and safe.
Place a cup of water in the microwave. Attach your amp probe to the line splitter, plug in and run microwave 10-20 seconds.
10-14amps = all is good
3-9 amps = transformer is good. Problem is in HV circuit. The magnetron is more likely to fail than the capacitor.
Under 3 amps the problem is in the LV circuit. There is only 1 diode in a microwave. It's inexpensive and simple to replace. To test the diode, I use a 9 volt diode tester. Honestly, if I'm replacing any part in the HV circuit I replace the diode too.
ALWAYS DISCHARGE THE CAPACITOR BEFORE DOING ANY REPAIRS
Unplug the microwave and once you remove the outer cabinet discharge the capacitor before doing anything else!
Be safe. The capacitor will kill and has killed many technicians.