For the Eames Lounge Chair, Vitra was not the original manufacturer. I would suggest looking for the Herman Miller label instead. While the point about the bolts is a poignant one, I have come across authentic examples of the chair with the exposed bolts. Some people have had problems with the arms coming loose on earlier chairs, so they had them modified with bolts screwed through. While I would never purchase one with a modification like this, I want to point out that it is still possible for it to be authentic. Again, I would say the determining factor is the Herman Miller label. Without it, there is a far lesser resale value.

For the tulip chair, the main factor is looking beneath the base. There should a marking stamped into the aluminum: BR-50 or BR-51. The Knoll label, if attached, will be found beneath the removable cushion.

The Barcelona is the tougher of the three to authenticate. Without the label it is very difficult for the average consumer to determine. Knoll didn’t always stamp their chairs with Mies’ signature of the Knoll label. Without the label on the frame, I would hold out for a label on the cushion. IF not, I would factor in the following: quality of upholstery/leather, owner’s provenance, context of the person/place buying from.