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]