Omega is a brand that is as well-known as Rolex.  It also has been replicated nearly as often as that other famous make.  Omega replicas come in varying levels of quality depending on the price range.  Cheaper Chinese copies are easy to spot, using poor metals and mineral glass for crystals.  Look at the dial printing to aid in detection.

Higher-end fakes are more accurate, such as the Swiss ETA-based replicas that use higher quality materials, including synthetic sapphire crystals.  These top-end replica Omega watches are much more difficult to spot mainly because of the increase in the quality of materials used in their manufacturer.

The following are photographs comparing a genuine Rolex model with its replica companion. You can see the two side-by-side, which helps clarify the differences. Note that there are a variety of fake Rolex watches on the market, all with varying quality. Some are better and worse than the examples shown below. Visit the Rolex Gallery for more pictures and information.