Smelling the cork in a restaurant will tell you if the wine is bad.

Corks usually just smell like corks and won’t give much indication as to the quality of the wine. The cork is only offered by the waiter for a quick examination to make sure that there isn’t any fraud (the cork matches the bottle) and that the cork hasn’t been compromised and/or allowed seepage in any way. Smelling the wine, on the other hand, is encouraged.