Microwaves within your microwave are invisible, but it is possible to see them with a little creative preparation and some mechanical know-how.
Says microwave modder Zeke Kossover:
The changing electromagnetic field from the microwaves will make charged particles move, and so the electrons in the metal legs will move creating current. This current makes the lamps glow. I drilled a grid in a piece of 1/4 inch acrylic and slipped the lamps in [...] As the platter turns, the lamps light up showing where the microwaves are the strongest.
Zeke also noted that this rig didn't seem to hurt his microwave. Caveat microwaver.
[Spotted on Make]