In todays world the only real way to tell is to buy it new from a trijicon dealer. One thing to keep in mind is if it sounds to good to be true, it probably is.
Fakes are a plague in the industry, and have been really bad for about 5 years. Its so bad, I do not accept trade in's anymore unless I have record of them buying that exact model from me. If they bought used, or from another company, they can trade in to those people.