WordNet (r) 3.0 (2006):

sniff out
    v 1: recognize or detect by or as if by smelling; "He can smell
         out trouble" [syn: sniff out, scent out, smell out,
         nose out]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "sniff out":
   be warm, burn, follow, follow a clue, follow up, get wind of,
   hunt down, nose, nose out, run down, run to earth, scent, shadow,
   smell, smell out, sniff, stalk, tail, trace, trace down, track,
   track down, trail