Well, according to the Diamond Guy (http://www.diamondcuttersintl.com/diamond_education/faqs.html) the best way is independent appraisal (basically, ask someone who knows real from fake). A fair fee is from $35 -$75 US for an appraisal. You want to be in the jewelers presence the whole time. Now, if you want to try some less reliable methods you can check this way: The old " if it scratches glass" method isn't as reliable as it seems. Many fakes also scratch glass. If you can read print through the diamond easily, it is less likely to be real, however, some shallowly cut diamonds can be read through as well. "Fog" the diamond wioth your breath. If it stays fogged for 2-4 seconds, it is fake. Real diamonds clears up almost immediately. However, oil and dirt can make this test less reliable. Cubic Zirconia weighs about 55% more than a diamond of the same size, however you need a carat or gram scale to weigh it. A large precentage of diamonds are blue under UV light. However, although this proves it is real, the bluer it is, the less valuable of a diamond it is. And the diamond guy also says: " Under the loop test. If you own some sort of magnifying lens, there are some things you can look for on the stone that might give away its identity: A. Look at the rock from the top and see how well the facets (cuts on top of the diamond) are joined. They should be sharp not rolled. B. Look at the girdle and see if it is faceted or frosty (a clear sign it’s a diamond) or waxy and slick (an indication it’s a fake). C. While you’re looking at your stone under magnification, look into your stone to see if you detect any flaws (carbon, pinpoints, small cracks). These are typically clear indications it’s the real thing since it’s very hard to put inclusions in a fake. D. After examining the stone, focus in on the stamps inside the setting. A stamp of "10K, 14K, 18K, 585, 750, 900, 950, PT, Plat" indicates the setting is real gold or platinum which gives a better chance that the stone in it is real as well. While you’re looking at the interior of the ring, also look for any "C.Z." stamps that would indicate the center stone is not a diamond."

why do you want to know? see a gemolist. or go to a jewelery store.

Take it to a reputable jeweler.