First off, let me say that the Planet Ocean (judging from the 'for sale' postings, spam, 'REPLICA' ads to which I've been subjected to recently), like the perennial favorite -- the Rolex Submariner -- is now one of the most counterfeited watches out there, so that's a red flag from the beginning; I see them all over, usually fakes by admission, sometimes not.
Now, some of this may be less useful to the novice, who has never really had the opportunity to handle the real deal for any length of time. Keep in mind that all of these are very basic tests, and all are NEGATIVE tests... they can tell affirmatively that a watch is a forgery, but won't tell you definitively that a watch is genuine. But, with a little know-how and ingenuity, you can whittle it down to all but the best Fake Omega Planet Ocean watches.
1. It should be shockingly heavy... far and away a hunk of metal like no cheap watch you've ever worn.
2. The crystal should be absolutely perfect and flawless... or darn close. Saphire is nigh-unscratchable, so a modern luxury watch should not show any wear to the crystal. If so, the owner should probably recall the incident with horror. A 'banged up' crystal is probably not saphire and thus the watch probably not genuine, unless the watch has suffered EXTREME abuse, in which case, run.
3. Set the watch crystal-down on a smooth surface, and gather the bracelet up on top of the caseback... give it a spin. The watch should spin like a top on the domed crystal... for a long time, due to it's weight and the tiny contact point. If it doesn't spin at all, it's a flat-ground crystal, and not a genuine Omega. An experienced eye can spot the difference in the reflection from the domed crystal, but it's not a skill for the novice.
4. Smear the crystal with water. The density, surface tension, and extremely fine smoothness of the saphire will cause the water to draw together quickly into a single mass (it's actually rather neat to watch), whereas it will stay completely diffused on a plastic crystal, and more or less completely diffused on a lower-quality mineral glass crystal. I've tried this with a number of watches with consistent results. The effect on a genuine saphire crystal seems almost magical.
5. Check the quality of the bracelet construction; The bracelet links should be machined out of solid steel, finished perfectly, with the edge of the bracelet polished to a mirror shine; They should NOT be made out of rolled steel or aluminum, or appear to have been layered and pressed; Many manufacturers hide this cheaper construction by peening/buffing the edges of the links to make them appear solid (For example, Fossil). Others, even moderately priced watches ($150+) use solid links very similar (though far less substantial) to the Rolex Oyster (Ex: Swiss Army), but most fakes are BAD fakes, and won't even bother with that. Modern luxury watches should use Solid end links (SEL) in the bracelet -- although there are a lot of better fakes out there with solid end links (the bracelet end link that contacts the case and through which the spring pins pass) -- it should be machined out of a solid piece of steel. Cheaper watches, including non-luxury types, and bad fakes of all kinds will have rolled and formed, completely hollow end links. This obviously doesn't apply to vintage watches, as even Rolex used these 'way back when,' but generally all modern luxury 'tool' watches (Rolex, Omega, Breitling, Tag Heuer) use solid end links.
6. Check for general wear. Luxury watches are made of sterner stuff than cheaper watches (including imitations); Wear in the bracelet should be VERY light and slight. This is very subjective and highly dependent on the age and amount of use the watch has endured. Generally speaking, a modern luxury watch, even with daily wear should look FANTASTIC for it's age... nearly new.
7. Dial, Printing & Engraving
A. This may be subjective, and you may need to be familiar with the genuine article to spot this, but quality can't be faked. A $4000+ wristwatch with a chronometer grade movement will have the same intense and unwavering attention to detail that one would expect in this range. And that means total perfection. Invest $3 in a 30x-magnifying jeweler's loupe, and scrutinize every detail of the dial, from indices, to patterns, to hands, certifications, trademarks, signatures: In the $3500+ range, every feature will display total crispness and absolute perfection and clarity, even at 30x magnification, AND even as applied over the wave pattern on a Seamaster dial. There should absolutely not be any trace of smeared ink, fuzzy edges to fonts, wavy borders to shaded areas, even on the miniscule 'SWISS MADE', and no included dust or other particles on the dial. Any of these displays an inattention to quality control that would horrify the quality control inspector of a fine swiss watch company.
B. The same applies to the caseback engraving (Hippocampus logo, Signatures, boasts of being the 'first and only watch...'); On a high-end watch, it will be crisp, perfect, and accurate, owing to the processes and quality controls used, usually machining or high-pressure stamping, whereas casting or lesser-quality stamping yields lower resolution, sloppier results. This is harder to tell without understanding the processes involved and their differing results. Suffice it to say that a fake Hippocampus or other relief caseback detail has a cheap and under-finished look to it.
8. Consistency in fonts and indices - It's well known that lots and lots of fakes still use old, plentiful counterfeit Rolex fonts on their date wheels, so a quick look at the '4', '6' or '9' can spot a cheap copy. Omega's fonts are quite different and unique. Also, Rolex-fonted, styled and indexed bezel inserts and Rolex styled applied indices are super-common in low-end fakes and counterfeits. This might not be the case for watches designed from the ground up to be accurate 'REPLICAs.'
9. The movement -- Even without opening the watch (which even I have not worked up the nerve to do yet), there are some things that you can tell about the movement inside. The quality and precision fit of the movement's functioning will set it apart.