Conventional method says to take a needle, heat it with flame and touch it to the resin. The needle will melt a small hole, sending a curly stream of plastic out of the hole and creating a plastic odor. The newest resins are said to resist this test. This is why we collect netsuke we enjoy. Too often and erroneously thought of as the only legitimate material for netsuke.
Elephant is the easiest to distinguish for the novice.
Elephant ivory has an obvious crisscross pattern grain on one of its planes. Because elephants are in danger of becoming extinct, the sale of netsuke of this material has become highly restricted. Mammoth ivory is, basically, fossilized elephant ivory. It usually has dark brown and or gray discoloration in the ivory because of the minerals picked up in fossilization. Because of its color, this is our favorite type of ivory.