You can tell if a diamond is real or fake by checking its edges. Real diamonds have sharp edges, while fake ones don't. Another way is to bring it to a jewelry shop, or have a professional look at it, which is the best and easiest way. You can also see if a diamond is real or not by placing it on top of a newspaper, and to see if you can read the text through the diamond. If you can, then it is a fake, because real diamonds contort light, making the letters of the newspaper blurry.
~Hope I helped!~