True ivory is made from the tusks of African and Asian elephants or from their very distant ancestor, the mammoth. Some people say the tusks of boars and the horns of narwhal should also be considered ivory, but there's some debate. When it comes to identifying real ivory, though, all you need do is follow a few simple steps and observe some definite signs.
Let's consider only elephant and mammoth ivory. There are some definite signs that you need to look for, but first there are some tools that you'll need. Get a bright lamp to make sure that you have a proper light source. You'll also need a magnifying glass or a jeweller's loupe so that you can get a very detailed look at the questionable piece of ivory that you're trying to identify. You'll also need some variety of needle or pin, along with some matches or a flame of some kind.
Once you have all of your tools assembled, it's time to take a close look at your ivory. Find a relatively flat section of the piece and then use your magnifying instrument to get a very good look. What you're looking for is a grain in your ivory. You should be looking for two patterns of lines; the first pattern will run in relatively straight, parallel lines while the second pattern consists of lines perpendicular to the first one. These intersecting lines should form tiny V shapes all along your ivory. These lines are called Schreger lines, and they're the identifying marks of genuine elephant ivory.
The Pin Test
If you didn't find a grain on your ivory, then chances are good that you don't have elephant, narwhal or even bone, but plastic. Plastic is a common substitute for ivory, and there's a simple way to find out if what you have is in fact made of plastic. Take the needle and your flame and hold the needle in the fire until the tip is red hot. Press the tip of the needle against your likely fake ivory in a place that won't show. Hold the pin there for a few seconds and then pull it away. If the pin melted a groove, then chances are very good your "ivory" is no such thing.