I use the tooth test when I'm buying from a source that seems a little shady. My teeth can pick up subtle textures that my fingers can't. It's not a garanteed accurate test but with practice you can use it to figure out if something is made of plastic, glass or something more organic. Try practising on various stones or beads that you know what they're made of to get a feel for the different textures. Again, I use this test when buying at a big gem show with venders from all over the world.