spotter

noun

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