The best test is to run the piece under hot tap water.
If it smells a little like your old high school chemistry lab (formaldehyde), it's Bakelite.
If it smells like vicks vapo-rub (camphor), it's Celluloid.
If it smells like burnt milk, it's Galalith (1920's).
If it smells clean, or like nothing at all, it's lucite, acrylic, or Lord knows what.