There are several ways. One is that a real rolex will not have anything engraved on the back. Rolex watches all have a smooth back. Check here http://www.qualitytyme.net/pages/rolex_or_replica.html for more useful info.

True story, I don't expect a rate. A few years ago, went on a dive trip to Cozumel. Done diving for the day, we headed into San Miguel to down some pints and check out the town square where the market is. A kid comes up to me, no older than 12 and offers me a Rolex for 80 bucks. I ask him how he can sell it for only that much. His reply. " Well, it's only 95% authentic rolex" I was laughing so hard I gave him 5 bucks for the chuckle. So there you go, a seller's honesty can tell you the difference.

A real rolex watch ticks on each second. a fake rolex watch has a smooth movement and does not hesitate on each second, as a real rolex.

This answer was written in responce to the original wording of the question: "How can you tell a real rolex from a fake rolex?" John Pennington has it backwards a real rolex doesn't tick and a fake one does. (I would have posted this as a comment, but the comment button ddidn't work at the time on my computer)

By the price.

The second hand on a real Rolex rotates smoothly, on a fake it jerks around (ouch sounds painful!).

Smash the shit out of it...if you get that sinking feeling of incredible loss in side your stomach, it was probably rea;l...

Take it to a jeweller and have him examine it. Or try and drop a cinder block on it. See if it breaks. If it doesn't, it's real.

A easy test is that a real diamond should cut glass. To be sure though you need an expert with a loop to tell.

you take it to a jeweler who can tell you if it is genuine or c.z.

If you have a good diamond simulant, then you really need to take it to a trained jeweler in order to distinguish between it and a real diamond. There is a myth out there that you can tell the difference between a simulant and a real diamond by attempting to scratch glass with the stone. Don't even think of trying this test if you value your stone. There are two reasons why this is a bad test. First of all, glass has a hardness on Mohs scale of only 5.5 (http://www.bbc.co.uk/dna/h2g2/A2982576). As you can see from the table given at the following link (http://en.wikipedia.org/wiki/Diamond_simulants#Summary_table), most of the good simulants have hardnesses that are higher than this. This means that most simulants will scratch glass. There are only two diamond simulants that are not harder than glass is, rhinestone and Strontium titanate. Rhinestone is not harder than glass because it is glass. Additionally, it is easy to distinguish from diamond because the back side (pavilion) has to be mirrored in order to give it the same sparkle as diamond. Strontium titanate is really not used much anymore because there are better simulants now. The second reason why this is a bad test is because diamonds have cleavage. Cleavage consists of one or more planes of weakness (cleavage planes) within the structure of a mineral. When a mineral with cleavage is placed under stress, then it will tend to break along these cleavage planes. Diamond has four planes of perfect cleavage. So, when you use a diamond to scratch a pane of glass, the stress that you place on the stone could cause the stone to chip or (more rarely) crack. In either event, this would reduce the value of the stone. (This, by the way, is also why the test DownOnMyLuck suggested is not a good idea.) So, trying to scratch glass with a gemstone to see if it is a diamond is a bad test because the ability to scratch glass is not unique to a diamond and, if it is real, you could damage your stone in the process of testing it. So, if you have doubts about the authenticity of a stone that someone claims is a diamond, then you should take it to a reputable jeweler and have him look at it. I also have one last piece of advice for you ladies. If you have reason to believe that the "diamond" that your man has given you is not real, then I would suggest that your relationship has more problems than a fake diamond. If you don't trust your man, then maybe you should reevaluate your relationship.

one way is hardness. a diamond has a hardness of 10. it can scratch mostly everything (even steal). chances are, most fake diamonds do not have that high of a hardness level...idk if that is any help.

One a diamond will cut glass; two diamonds have flaws fakes don't!

I know of one easy trick. If you buy a diamond with inclusions in it then it is invariably always real. This is simply because its really hard to put such flaws in synthetic diamond. I think someone said something about fluorescence under a UV light but I can't remember if that was for or against it being real.

The only people who can authenticate a real diamond are certified gemologists specializing in jewelry. However, if you are browsing rings at a flea market and want to quickly assess whether a clear stone is probably glass, cubic zirconium, quartz, or leaded crystal, there are a few easy tests you can apply without any special equipment. These tests at least rule out recognizable imposters because they rely on the way a real diamond stone refracts light, conducts heat, and looks up-close. If the gem in question is loose and unmounted, try placing it over printing. Diamonds refract so much light that they will not work as a magnifying glass and you would see no lines, circles, or letters through them. Other clear stones like glass or crystal will reveal the print clearly. A similar test uses a small light, such as the one that comes with your key ring. If you shine the light through the stone and cannot see it on the other side, but only a bright halo around the rim, it is more likely to be a true diamond. Diamonds are also very good at conducting heat. Breathe on the surface of the stone and immediately check to see if it has fogged up. Again, quartz, glass, and cubic zirconium will stay hazy for a moment before the condensation dissipates, but you shouldn't be able to see any moisture on a true diamond. However, one rock, called moissanite, will also pass this test, so the best way is to get a complete thermal conductivity evaluation performed by a jeweler. Looking closely at the specks, ridges, and facets can also give you a clue into the identity of your gem. Diamonds do have inclusions, tiny bits of other minerals that got crushed into the diamond while it formed deep under the earth. However, if the facets have worn or rounded edges, bubbles, or if the gem looks rippled or pitted, it is probably glass. Those gems that are perfectly clear, with absolutely no inclusions, are probably quartz. Finally, some common sense measures can prevent you from getting pressured to purchase a gem that turns out to be something other than a diamond. It shouldn't be ridiculously inexpensive, or you'll get what you pay for. It also will usually be mounted in an "open back" setting, which means that you can see around the rear of the gem and the back surface isn't coated with any silvery substance. Although it's true that diamonds are the hardest organic substance on earth, the infamous test of scratching diamonds across glass or metal might just give you a damaged diamond. Even many gemologists cannot distinguish "real" diamonds from cultured diamonds. Cultured diamonds have been artificially manmade in a laboratory, not mined from mountains, yet they are chemically identical to those diamonds. Some companies are developing new kinds of identification methods for those who would like to purchase a mined diamond or for those that prefer a cultured one. http://www.wisegeek.com/how-can-you-tell-if-a-diamond-is-real.htm

If the diamond cuts glass, it is real. This has been a tried and true example of a real diamond for many, many years. And, its a cheap test for the truth.

its really really difficult for an untrained person to tell. your best bet is to take it to a gemologist or a good diamond retailer. www.americangemsociety.com has all the info on real diamonds and certification of diamonds. I would definitely take a look at it.