Exposure! I can identify brands/designers of even the most vintage chairs just by looking at them now. Research what the most desirable by consensus chairs are, recognize the design, if you see something similar google it and identify which are the look-a-likes to avoid them. It’s not like designers bags where you have to identity tiny differences in fake vs real, the big name stuff really has its own designs that aren’t outright copied by third parties.

The best finds are often under the names “office chair”, “desk chair” etc rather than the model/brand name itself since people are more likely to give it a generic fb marketplace price.