If your microwave oven is broken, it might be because the diode has gone bad. If you think that may be the culprit, you might want to test it. In the hobbies world, high-voltage diodes are often used to make high-voltage power supplies, but if you take it from a broken microwave oven, this video will show you how to test the diode for proper functionality using a direct current (DC) power supply.
This shows how to test if a high-voltage diode is good and how to determine the direction of a high voltage diode, in case it's not marked. High voltage diodes can be bought retail, but are more commonly found in microwave ovens.
A normal meter by itself can't test high voltage diodes. For that you need a DC power supply and some resistors, as this video illustrates.