To check it unplug the unit, discharge the capacitor using two screwdrivers with the plastic handles (cross screwdrivers touching both capacitor's contacts).
Using the multimeter check the resistance, first between the contacts (with all wires removed from the capacitor). On the analog meter you should see the spike followed by infinity.On the digital-no continuity.
Next check each contact to the ground. Should be no continuity.
Replace the capacitor if it's bad.

Here is the breakdown diagram and Microwave parts for GE JVM1740DM2WW - AppliancePartsPros.com

Post the results.

Simon.