The Collaborative International Dictionary of English v.0.48:

Sleuth \Sleuth\, n. [Icel. sl[=o][eth]. See Slot a track.]
   The track of man or beast as followed by the scent. [Scot.]
   --Halliwell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sleuth
    n 1: a detective who follows a trail [syn: sleuth,
         sleuthhound]
    v 1: watch, observe, or inquire secretly [syn: spy, stag,
         snoop, sleuth]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "sleuth":
   Bow Street runner, FBI, FBI agent, Federal, G-man, Secret Service,
   Sherlock, Sherlock Holmes, T-man, detective, dick, eye, fed,
   gumshoe, hawkshaw, hotel detective, house detective, house dick,
   inquiry agent, investigator, narc, operative, plainclothesman,
   police detective, private detective, private eye,
   private investigator, revenuer, shamus, snoop, store detective,
   tec, treasury agent