Identifying Barcelona Chairs  

pictures help
Some key factors are dimensions. Knoll labels on the underside of the cushions. The number of straps and whether or not the cushions are tufted front to back, or just front only. Knoll didnt start stamping signatures until the mid 90s I believe.

Yes, mid-90s.
Signatures started in 1996.
The first post in this thread went unanswered for almost two days; I suspect that that's because some of the replicas are VERY similar to the Knoll chair, so it's hard to give identification advice. Alphaville's version of the chair, for example, is said to be practically indistinguishable from Knoll, even side-by-side (which is why Knoll and Alphaville are currently in court).
It used to be that you could tell a replica because it didn't have the right number of straps (9 on the seat, 8 on the back) or the right number of tufted squares (5x4 on each cushion), or because the cushions didn't hang over the edge of the frame, or the straps didn't attach the right way or the frame-ends were ground wrong or the welds weren't smooth enough, or just by taking a tape-measure to the damn thing. I don't think that's true anymore.
These days, I'd be very nervous about paying more than the cost of the lowest-priced decent replica ($700 or so) for any chair represented as pre-1996 Knoll unless I had an expert with me to authenticate it.

Thank you for the suggestions.
Do you know if there is more information on the subject available on the Internet? Or, perhaps, in the library?

Consumer
If you email Knoll directly, they can authenticate it for you. A few years ago, I emailed them a photo of what I thought was a real Saarinen Executive rolling desk chair. A customer service rep emailed me back to tell me it wasn't Knoll and just a very good copy, and showed me photos from their product archives and told me how to tell the difference.
It took a couple of weeks for the reply, but in the end they were very helpful people!

While you're waiting for a re...
While you're waiting for a response from Knoll, you could compare measurements of your chair with that of others. Some auction houses/dealers provide exact measurements of chairs they have/had.

And I would measure
Everything. Width of the bars, thickness, number of straps, width of tufting, height from the feet to the frame etc etc.
My guess is that, if these chairs are older copies, they would exhibit some of the more obvious flaws in construction than the newer ones FstFwd puts forth.

I looked at a set of ...
I looked at a set of Barcelonas last week and determined that they were a reproduction from the early 60's , they had 9 straps knoll 8. a magnet stuck to the metal. so they were not stainless. no buckles on the back, fully covered in leather no cloth, like knoll on the bottom cushion.
I passed and glad i did, they would have cost over a thousand dollars to recover.