You can tell whether a diamond is real or fake by looking at certain details of the diamond, such as presence of imperfections, the shape of the edges and the stone’s refractivity. Other tests include the "fog test" and attempting to scratch the stone.
To look at the diamond’s details, you will need a magnifying glass. Since diamonds are natural combinations of carbon, they tend to have some imperfections. A real diamond will likely show small imperfections, while a fake stone may have no blemishes at all.
Since diamonds are cut, they tend to have sharp edges. A fake diamond usually has rounded edges instead.
If the stone is not in a setting, you can check its refractivity by placing it over newspaper. A real diamond will scatter any light going through it, preventing the black reflection from the letters from showing.
The fog test consists of breathing hot air on the diamond. Since a diamond does not retain heat, it will not fog up. A fake diamond will retain fog. Lastly, you may rub sandpaper against the stone. A fake stone will scratch up, while a real diamond will not.
For assurance of a diamond's authenticity, an appraiser at a jewelry shop can provide certified assurance.