Can you tell if you have decayed tooth by the smell?

No.: Most decayed teeth have no smell. However, if the decay is really deep infecting the nerve of the tooth, the resulting infection ( abcess) can definitely have a smell.

No: As mentioned above a cavity typically does not have a smell. If you are experiencing a foul smell, it is likely a sign of an infection. Often this is a result of an acute abscess or chronic periodontal disease. Ther are many other causes of halitosis...Bad breath...So it is best to see a dentist for an evaluation.