ivory
On any substance you think might be ivory find a place that is in an area not noticed and you can take a pin heat it and touch the material. Ivory will not be effected by this an a mandmade substance will get a small burn mark. Maybe do it on the face of a joint right by the wood so you couldn't see it later. Thaat is a sure fire way to tell. I have worked with ivory for over 25 years and have used several tusk myself and have seen some substitutes that were very close but this method works.
Percy