the 'magnetron' ,the thing that generates the microwaves,is eitheron,or off, when you are using it. It doesn't workon ,eg. 'simmer power' the magnetron is simply switched on and off-the off periods being longer when a 'lower power ' setting is selected. If the magnetron is generating microwave energy at all ,then its OK To test put a small amount of water in a cup and five it a few minutes- if the water heats ,then all is OK.
Microwaves only heat water molecules, so if heating plates for example,ALWAYS put a little water between the plates.