There are indeed some very, very good fakes.
I saw some fake Hublots in Thailand that were mroe or less indistinguishable from the real thing - maybe they came out the back door of the same Chinese factory. OK, ok, it's probably not that hard to fake a Hublot!
BUT...
A friend and fellow enthusiast purchased a used Planet Ocean from a well reputed jeweller in London. He sent it to Omega after a year or two for servicing, only to be advised by Omega that the PO was, in fact, fake. He took the watch and letter from Omega back to the jeweller, who was extrremely embarrassed and refuinded his money quick smart.
So...
BUY THE SELLER. Even then, no 100% guarantee unless the seller is an AD, but still, that is your best bet.