spotter in WordNet English dictionary

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