I don't mean this to come off as sounding like a smartass but you can just tell. The fake ones have some kind of garbage chinese black spray paint on them that from what I've heard (never personally handled a fake one) comes right off on your hands. On the fake ones, the part where it says "Norotos" and the PN are much fainter or not present at all, and often the fake ones would not include the adapter to use the USGI Rhino arm. As far as the rubber gasket, I believe that may only be on the newer ones. I know the Universal shroud has a gasket but the normal MARSOC/WARCOM 3 hole may not, especially if this one is circa '07 as the auction states. Think about it. If the seller was trying to pass off a knock off, he probably wouldn't provide several highly detailed pictures of the item.
I'm sure some european civilian gear queer will come in here and blast me and say that you can tell by some little screw or something.
But I've seen plenty of them, and I would put my bottom dollar on that one being authentic.