Very high-quality forgery can only be discerned by professional gemologist (jeweler). As a rule, to the naked eye, without special tools it is almost impossible. But you can detect obvious forgery you can own, knowing a few proven techniques. To determine the authenticity of the stone you will need newspaper or any printed text on paper, a small flashlight with a bright beam of light and a container of clean water.
If the diamond you buy is not cut, then put it on the printed text and see what you can see through it. The fact that the real stone is very strongly refracts the light and you will not be able to distinguish the printed text, that is all very vague. Using fake stone letters are well mapped. Also you can check the refraction of real stone with light. Take a small flashlight and point it at the stone that the beam passed through him. If you do not see light on the other side of the stone, the diamond is likely real. Light, due to refraction, should dissipate.
Try to get some on the test stone. Since diamond conducts heat superbly, you will not be able to see it obscured the trail. But not only a diamond is capable of good thermal conductivity, so this check stone better to do with your jeweler.
Now inspect your diamond on the edge. There are natural diamonds and artificially grown. Artificial gemstones are usually much cleaner than the real thing. In the water they will be almost transparent. Real, mined from the earth, on the contrary, will have various small inclusions. But not bubbles, as the air in the stone, evidence of apparent forgery.
Despite the fact that diamonds are the hardest substance on Earth, they better not cut the glass in order to authenticate. This can significantly damage the stone.
Please note the price of the jewelry. A real gem can't be cheap.