The plastic, being crystalline, messes with the polarised light passing through it and generates quite a light show. With the help of a polarising microscope you can use the same technique to find out whether your insulation is fibreglass or asbestos. New Zealand a. Teeth are the best way to distinguish between glass and plastic, using a combination of feel and sound. Gently knock the object against an incisor.
Plastic produces a dull thud, while glass gives a high-pitched clink.
Wristwatches are a good test for this theory because some have a glass front and others plastic.