You can test this quite easily with something called a triangle test. Heat some milk in the microwave, and heat some more milk on a gas stove. Pour two cups of one heating method, and one cup of the other heating method. Line up the three cups, and remember which is the odd one out. Then, ask a family member to come into the kitchen, try all three, and ask them which is the odd one out. If you try this a few times, then if there is a difference, they should be able to correctly state which is the odd one out most of the time. If they can’t tell a difference, then they’ll get it right about one of every three tries since they’re just guessing.

Or you can do a rigged triangle test where all three are heated by the same source, and then when they get confident in their ability to say which was microwaved, you tell them they were all done on the stove.