Tap the crystal with the back side of your fingernail. If it is acrylic, it'll have a high pitched sound and feel hollow. Mineral glass will be more dense and have a lower sound. Sapphire will be a 'thud' and feel very dense. Easiest way I know, but it helps if you have different types of crystals available so you can tell the difference between them. If it's acrylic, it'll feel like and sound like your tapping on a thin piece of plastic.