How can I recognize a Wassily chair?  

that's incorrect, martin
My vintage Knoll Wassily chair has 3 hex-screws visible under each arm, along the chrome tubes.
Best,
Mark

Thanks everyone for the...
Thanks everyone for the feedback. These are definitely authentic Knoll chairs from sometime in the 70's. The leather is very thick and I believe it is referred to as "belt" leather.
Any idea what original chairs such as these sell for currently? Aside from the leather being a bit stiff they are in nearly new condition.