The Collaborative International Dictionary of English v.0.48:

spotter \spot"ter\ (sp[o^]t"t[~e]r), n.
   One who spots.
   [1913 Webster]

WordNet (r) 3.0 (2006):

spotter
    n 1: a worker employed at a dry-cleaning establishment to remove
         spots
    2: a worker employed to apply spots (as markers or identifiers)
    3: a person employed to keep watch for some anticipated event
       [syn: lookout, lookout man, sentinel, sentry,
       watch, spotter, scout, picket]
    4: someone who is the first to observe something [syn: finder,
       discoverer, spotter]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "spotter":
   Argus, CIA man, Charley, airplane spotter, beagle,
   cloak-and-dagger operative, counterintelligence agent, counterspy,
   dick, double agent, espionage agent, eye, fire patrolman,
   fire warden, fireguard, flatfoot, forward observer, gumshoe,
   gumshoe man, hawkshaw, inside man, intelligence agent, lookout,
   lookout man, military-intelligence man, naval-intelligence man,
   night watchman, operative, patrol, patroller, patrolman, picket,
   private eye, reconnoiterer, roundsman, scout, secret agent,
   sentinel, sentry, skip tracer, sleuthhound, spy, spy-catcher, tec,
   undercover man, vedette, watch, watcher, watchkeeper, watchman