Zeke Kossover built a grid of tiny neon lamps inside a sheet of acrylic. He explains how this will work:
Microwaves are invisible, so you can't see them inside microwave oven, but their presence can be detected with neon lamps. 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.
One thing you notice watching the microwave do its thing: the microwaves don't always spread evenly, some spots absorb more microwaves than others. That's why sometimes food can be ice cold in some spots and scorching hot in others after you microwave it. [Hands On Science via MAKE]