Fill the glass measuring cup or microwave-safe dish with cool tap water. Put the measuring cup in the microwave.
Place the dish to be tested next to the measuring cup in the microwave. The two should be very close together, but not touching.
Microwave the items on 100 percent power for 1 minute.
Use a potholder to remove the measuring cup and the tested dish. Lightly touch the dish. If it is cool, it is safe to use in the microwave. If it is warm, it can be used but only for very short cooking times. If it is hot, the dish contains material that absorbs heat rather than allowing it to pass through the dish. That means it is not safe to use in the microwave.