There's an easy(ish) way to clean one. Put about 1-1.5 cups of water into a microwave safe bowl or glass (I use Pyrex measuring cup) and microwave it for about 10+ minutes. Let the water boil really good and the hot steam will soften all the crap on the inside of the microwave. Get the cup out carefully, wipe the inside with a wet cloth, maybe spray some cleaner if oily and you're done.
You really want to let that water sit still for a bit before you take it out. It could have superheated (meaning a portion under the surface tension has converted to gas) and explode when disturbed.
Be very careful doing this. The water can become superheated and explode when the surface tension is broken. Honestly, it's probably better to find an alternative way to clean your microwave.
There's nothing special about a microwave that will superheat water. You can superheat water on a stovetop, but nobody ever says not to boil water on a stove.