Fake Planet Oceans are easy to tell, a lot of signs. HE Valve is in a slightly different position, usually no AR coating, the hands are a bit shorter on some fakes, the Omega applied logo is wrong and the hour markers are thinner. The orange usually isn't as deep, rather its just one shade of orange. The pearl is usually way off as well.
Very sadly the Planet Ocean turned out to be a replica
It was only when one of the guys I know well in my favourite jewellers (I bought the Carrera and two Oris from him so those are beyond reproach) took a look at it a few months ago. Even he thought the Planet Ocean was an original until he compared it to an original he had in the shop. So unless you are in the trade and have read up extensively on the fakes being produced it is almost impossible to tell without having an original one to hand. What made it even harder to tell when buying it was that the strap, box and papers it came with were probably genuine, might have been stolen from the factory which lent it some authenticity. The galling thing is that the price, while good (obviously), was not commensurate with it being a replica.
Never mind, you live and learn and at least there was nothing fake about her other Christmas 'presents'