CZ (also known as industrial diamonds) are usually flawless, so if you have a good jewlers loop you can tell the difference between fake and real. Also issues of color can give you a good idea which is which. Ultimatly, most jewlers or diamond buyers have a diamond testing laser, a small pocket device that will indicate if a diamond is natural or not.
Natural diamonds are worthless IMHO. It was only until the early 20th century that they became popular in wedding rings. A brilliant piece of marketing by the Debeers company, since they control 2/3rds of the worlds diamond supply, and stricktly control output to keep prices high.
Wholesale diamonds are remarkably cheep. Aside from the quality of a diamond, a diamond that sells for maybe 450 bucks wholesale goes for maybe 1,500 to 2,500 on the retail market. How else do you think they can have a 50-75% off sale and still make a good profit. The markup is so good its evil.