I don’t think i’ve ever seen a knock off NF scope, but i imagine you can tell pretty quickly whether it is legitimate or not from the finer details of the scope such as the turrets or the laser engraving.
You must have gotten this scope for a super deal if you think it’s a fake... well done. Like the above said I have never heard of a fake. Next thing you may need to worry about is that it could be stolen. Highly doubt it but I feel like that is more likely than a fake.