First, look at the setting and mount. Since stones such as cubic zirconias are so much cheaper than diamonds, they are sometimes set in inferior metals. Then check the wear and tear on the stone. A cubic zirconia is not as durable as a diamond and may show signs of scratches or nicks. No matter what you see, you should take the stone to a trusted jeweler. Looking through a jeweler's loupe, he or she should be able to spot a fake stone right away. If the jeweler cannot tell by examining the stone, he or she will have instruments that can verify its authenticity. http://www.pointask.com/pointask/f_q.php3?qid=23036

What to look for depends on what type of gem stone you are talking about. To distinguish between synthetic and natural diamonds, you need to look at the inclusions. Even the best natural diamond will some other minerals grains inside of the stone. Synthetic diamonds, on the other hand, will not have mineral inclusions. Instead, they will have primarily gas-filled voids as inclusions. Inclusions will show the same pattern with the colored gemstones (rubies, sapphires, and emeralds) as they do for diamonds. However, colored gemstones have something else for which you can look. If you look at a synthetic colored gemstone under magnification, then you will often see that there are variations in the color of the stone. These variations will usually occur as curving bands. This reflects the way in which synthetic gems are created. The process grows them as conical or cylindrical boules. I can't remember if natural stones will have banded color variations*, but if they do, the bands would be in concentric hexagons (or parts of hexagons depending on the cut of the stone). This would reflect the way the stones grow naturally.