There are multiple tests to find out about the same. Some of the easiest ways to check if the diamond is real are:
Place the stone close to your mouth like a mirror, blow over that to fog it up with your breath. If the stone stays fogged with the breath, it is probably a fake diamond. A real diamond won't fog up easily.
There is a test called the sparkle test, which is very easy and quick because all it needs is your eyes. Hold the diamond under a normal lamp and look for the bright shimmers of light that will be reflected and bounce off the diamond. If a diamond is real, it will have exceptional light-reflecting properties. A real diamond is high in density, and it will sink to the bottom of the glass. This is called the floating test.