Test with Fog
To tell if your diamond is real, place the stone in front of your mouth and, like a mirror, fog it up with your breath. If the diamond stays fogged for a few seconds, then it's probably a fake. A real diamond won't fog up easily since the condensation doesn't stick to the surface.
Test with Water
This one's easy: Get a glass and fill it with water (it doesn't matter what type of water you use). Drop the diamond into the glass of water. Due to the high density of diamond, when dropped into water a real diamond will sink. If the diamond floats to the top or middle of the glass, it's fake. It's simple physics.
Test with a Magnifying Glass
To determine if your diamond is real, hold a magnifying glass up and look at a diamond through the glass. Look for imperfections within the stone. If you're unable to find any, then the diamond is most likely fake. Real diamonds have imperfections referred to as inclusions.
Test with a Black Light
You'll need a black light for this one, obviously. Once you have it, turn off the lights and hold the diamond in front of the black light. Most diamonds will reveal blue florescence under a black light; therefore, you'll see a medium to strong color of blue, which means the diamond is real. If you don't see the blue color and instead see a slight green, yellow or gray fluorescence then this usually indicates the gem is not a real diamond. But remember: This is not a conclusive test and not all real diamonds will reveal a blue fluorescence.
Test with a Loupe
A loupe is a small magnification device that's used by jewelers to see small details up close. The difference between a loupe and a magnifying glass is simply that a loupe doesn't have an attached handle and its lens frame is slightly more conical. While looking through the loupe, look for inclusions. Mined diamonds usually have tiny, natural imperfections which indicate the diamond is real. Next, check to see if there are small flecks of minerals or slight color changes. If there are signs of slight color changes and flecks, then the diamond is more than likely real.