I think the big question is whether you are happy with the way the clubs play. I have had a number of iron sets over the years and some of the cheapest clubs i have had were some of the best feeling and best hitting clubs. Some of the most expensive ones I have had were the worst feeling and worst hitting clubs.
While counterfeiting of clubs has become a much bigger problem that most people realize, with the advances in technology most iron counterfeits or knockoffs have the very same specs and material as the "real thing" and many of the heads for irons are actually made in China--probably at the same factory that made the knockoffs.
Unless you are a scratch golfer you probably cannot tell the difference--and you may not be able to then. Perhaps one way you may be able to tell is whether the seller can give you the specs of a particular iron. Most of the people trading in counterfeits have no clue what the swing weights , lie angle or loft angle are for their clubs.
With the move by some companies to hollow core irons that contain varying amounts of foam (e.g. TMs P-790 or Pings I-500 or G-410 or 700 it becomes more difficult to counterfeit because the amount of foam or its density is a very closely held secret that can't be copied simply by cutting a club in half and measuring it.