sentinel vs spotter

sentinel and spotter both are nouns.

Word	Noun	Adjective	Verb	Adverb
sentinel	Yes	No	No	No
spotter	Yes	No	No	No

As nouns, sentinel and spotter are synonyms defined as:

sentinel and spotter: a person employed to keep watch for some anticipated event

Other synonyms of sentinel include lookout, lookout man, picket, scout, sentry, watch.

sentinel (noun)	spotter (noun)
a person employed to keep watch for some anticipated event	someone who is the first to observe something
	a worker employed to apply spots (as markers or identifiers)
	a worker employed at a dry-cleaning establishment to remove spots
	a person employed to keep watch for some anticipated event