OK.
If you ask a specialist for paintings to tell you how to tell the difference between a real antique painting and a professional reproduction you may know the theory, but you will never be able to tell the difference unless you have seen enough originals and fakes.
Same with rugs.
It is not hard to tell on such a poor fake like on this rug. Any carefull viewer can see the difference in wool quality surrounding the letters and the rest of the rug, also the different color. It is not a bad Nain, and I also thnk it is a 6 la. But if someone paid 330k for it, it is too much. I would love to have such customers :-)!
Some things to remember:
Habibians are very expensive (this is why they are oject to be faked). Usually you don`t find them on auctions (unless they are fake), on ebay or any other whatever great opportunity to get a rug cheap.
You find them in real good stores where they cost the price of a Habibian.
PS. It is not a Habibain design.