I'd be happier with a simple ohms test across the capacitor, showing that it's S/C - doing an AC test at a frequency far above it's designed usage 'may' perhaps give a false reading?, and there's no advantage in doing so.
I've repaired hundreds (probably thousands) of microwave ovens, and the capacitors do very occasionally fail - however, it's also quite possible that it takes out other components as well. It rather depends on the actual design, later ovens have protection diodes (a seriously HV zener), and these are designed to go S/C if the rectifier or capacitor fail - on older models failure of the rectifier or capacitor doesn't always blow the fuse, resulting in the transformer dying. The protection diode ensures that the fuse blows, and protects the transformer.
As others have said, the easiest place to get one is out of an old microwave, they fail very rarely, so an old faulty microwave if most likely to have a good capacitor you can remove. The value isn't critical, they are all pretty well the same.