Question

How do ketones smell?

Answer 1

With their noses. ;)
OK, seriously, Aldehydes and ketones are known for their sweet and sometimes pungent odors.

Explanation:

See http://chem.libretexts.org/?title=Core/Organic_Chemistry/Aldehydes_and_Ketones/Properties_of_Aldehydes_%26_Ketones/Natural_Occurrence_of_Aldehydes_and_Ketones
for further descriptions of specific compound odors and molecular structures.