You can detect the presence of the wave energy by noting their effect. If you put something which contains fat or water in the microwave it gets hot. That demonstrates the presence of electromagnetic energy. There are quite a few websites that discuss why this is. https://sciencequestionswithchris.wordp ... -to-water/
seems like a good one and it discusses the common myth of why microwaves run at the frequency they do.
You can also measure any energy that makes it past the door with a special meter. I know you just asked your question two days ago and the project is due tomorrow. That's not enough time to get a meter but you can find quite a few with an internet search. A good one will show you the energy that makes it past the door seal (you don't want to see a lot, but you will likely see at least a little). That would demonstrate that energy, and hence waves, is present. It also tells you why it's not a great idea to put your face up the window and watch things cook.
I hope you were able to figure out a project in your short time frame.