How, specifically, does an electric meter measure wattage?

Are you allowed to make a Tesla coil? You wirelessly power something if it was close enough. I'm not sure if you will be able to measure the current or not. If you think about it, the Tesla coil would be the power source and what your powering would be the load, and since they are not directly connected, you might not be able to measure any current.

It's just an idea.