Being in the business and all, you would think that I could tell, but I'm telling you, it's not easy. Not with a magnifying glass, not with a loupe, not with superman sight. There are many different diamond grades based on color and clarity. The best way to spot a fake is to find one that you don't see any imperfections in. For real diamonds of super high quality, there will be less imperfections, lower grade, more imperfections (or, as they are called, inclusions) Another way to spot a fake is if it has a scratch on it. Diamonds don't really scratch so easily
(only by another diamond) That is one of the reasons they make great engagement rings. You can wear them all the time without worrying about scratching the stone.
I recently priced out a 1.10ct stone for a friend that was of really high quality, GIA certified, and the cost for just the stone was $21,000 wholesale. To buy this stone set in a simple setting with no other diamonds, 14k gold, would be in the $30,000 range retail!!!
I don't have a diamond in my ring. I have a tsavorite. Not as hard as a diamond, but it's my favorite stone.