The solutions lies beneath
I believe that earlier objects had a number printed into the wood under the rounded 'feet' (runners or armrest so to speak). If it is truly old and worn it might be hard to see as it has been chafed against the floor. One of my lounge chairs vaguely says '42'.
I think with some designs,...
I think with some designs, this would be a good example, you'd be on pretty safe ground, especially if its obviously an older piece. The high investment in tooling for what is a relatively low selling product would deter the rip off merchants.
Its slightly different now with the good cheap cnc cutting for mold work and the Chinese capabilities but you can see even there that the designs that are copied tend to be the more easily manufactured or those with high prospective sales where the investment can be recouped (eg Brno and Tulip chairs).
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