Check to see if on the caseback the word "WATERPROOF" has an "A" with a flat top or a pointy top. Flat-top is good, pointy-top is BAD. 
 
  

Check to see if on the caseback the word "CERTIFIED" is spelled correctly. Check the spelling of ALL the words on the case back.

 

Check to see if the case back has "A/254/67" engraved *anywhere* on it. This is BAD. There are other fake case back markings too, but this is the most common.

    

Check to see if on the dial the MINUTE MARKS happen to extend past the outer edges of the HOUR MARKS, if they do this is BAD.

An example of a BAD DIAL, see how the minute marks stick out behind the hour marks at 9,10,11



Two examples of a GOOD DIAL(s)

 

An example of a BAD DIAL, but this time a DATE DIAL, with no box around the date, AND an example of a GOOD DATE DIAL, see the little white box around the date (date itself can be either white-on-black or black-on-white).



ANY of these is an indicator of a FAKE WATCH, you don't need to have them all.

The fact that the watch most likely does have a genuine Omega movement does not make it real, as the correct Omega movements, while being of very high quality, are very common.

http://www.dealingtime.com/upload/files/65/Stubbies-Attack.jpg