How to tell if it's Ivory or Plastic?

The best way to tell whether your item is not plastic and is in fact ivory is with the Pin Test. This is done by heating a pin until it's red hot and placing it on the item where you don't mind placing a small burn mark. The smell that the ivory gives off is similar to the smell when your dentist has a drill on your tooth. If this is done with plastic or any other man-made material you will not experience the hot Pin Test.

Now that you've eliminated the possibility that your item is plastic you have to determine whether it's Elephant Ivory or some other bone?

The best way to tell if the your item is elephant ivory is the characteristic of the cross graining or criss-cross growth patterns distinctively found only in elephant ivory. No other mammalian ivory has this characteristic. These growth marks or cross-hatchings are called Schreger Lines. Schreger lines can be divided into two categories.  The highly visible found on the outer portion of the elephant tusk and the fainter lines that are found near the root or the inner portion of the tusk.  This explains why sometimes the growth patterns are not very visible.  Seeing the grain is also dependant on whether the ivory was cut out against the growth grains.  The growth patterns run the length of the ivory tusk. The best way to explain it is imagining a piece of wood. When you cut wood in a perpendicular fashion relative to the tree trunk, you will see the growth rings of the tree.  However when you cut in the same plain as the length of the tree trunk then the grain is no longer as visible. This is the same concept of the visibility of elephant ivory cross-grains, sometimes you see it and sometimes you don't. Usually you have to look at the width or thickness of the piece to see any cross-graining. However sometimes the piece was extracted out against the grain and the growth patterns are highly visible throughout the piece.