Yes, take it to your local Omega AD. Even a non-Omega AD who deals in watches in general can usually tell simply by unscrewing the back (even if it is a sapphire display back) and verifying it's the real thing. Check all serial numbers, on the back of the case (lower left-hand lug) and on the movement. And yes, non-ADs sunning themselves under 'grey' skies can sell them, you get everything but the original manufacturer's warranty (I believe Seiko and Citizen are the exceptions), but you will get the seller's warranty, whatever that's worth and really covers. You also don't know if you're getting a factory reject or a slightly used, second-hand watch, etc. Caveat emptor, yadda, yadda. Real Omega and Rolex watches are tanks, so it's up to the buyer where to buy 'em from. Enjoy! And Happy Fourth of July here in the Colonies! ROCK!!
=:^D