How do you tell if your crystal is diamond?

The best thing to do is take it to a reputable dealer in town, unless you have a great deal of experience in telling whether a diamond is genuine or not. Also, if you have a diamond that you KNOW is genuine, you can place both of them, side by side, on a black cloth & compare the 2. It is pretty obvious to the naked eye the difference when they are side by side - but if you are just looking at a single diamond, sometimes it can be tricky.