This does remind me about an invention… well, more of a hack or project than an invention, which is something you put in a microwave as evidence that it is working. Isn’t that just a cup of water? Heat it and check it? Yes, that works, but you can’t look in and see an indication that the magnetron is on. Say that like me, you like to fiddle with the power setting because you believe (with poor justification ?) that certain foods should be reheated more slowly. Does the change in the sound of the microwave mean the magnetron is off, or has gone to lower power, or is it just the fan, or what?
So why not make a little antenna connected to some led’s that shows the current microwave flux ? Throw it in with your burrito and watch the light show. Be the first one in the household to know for certain, yes, that loud bang means something happened to the power supply of the magnetron.
And of course, convince biology teachers that it is safe now to open the door.