Question

A friend tells you that a refrigerator door, beneath its layer of white-painted plastic, is made of aluminum. How could you check to see if this is true (without any scraping)?

Answer 1

A metal detector will show if it is metallic, but will not identify the metal. A magnet will not work.

You could get the density of the door by taking it off its hinges and weighing it and measuring its volume. You could also infer the mass from its moment of inertia on the hinge. This would require some mechanical measurements.

Measuring the X-ray penetration or measuring X-ray fluorescence with would also work.