This article shows a rather easy, albeit not very accurate, way to test if a microwave is leaking radiation:
http://www.iflscience.com/technology/how-tell-if-your-microwave-leaking
How hard would designing a special circuit for this application be?
http://www.iflscience.com/technology/how-tell-if-your-microwave-leaking
How hard would designing a special circuit for this application be?