When holding genuine crystal glass up to the light, a rainbow should be seen because crystal glass acts as a prism. If a rainbow cannot be seen, it is plain glass. Crystal produces a musical ring when it is struck, and glass does not.
A crystal glass also creates a musical tone when a wet finger is ran around the rim. Crystal is worked thinner than glass, so a glass with a very thin rim is probably a crystal glass. Crystal glass is heavier than ordinary glass of the same size. Crystal typically has clean, rounded cuts, while glass has sharp cuts.