spotter

Short definition from Wiktionary

Noun

1. 1
   A person who observes something.
2. 2
   A member of a sniper team who in addition to this function is responsible for providing additional information about targets from a different point of view.
3. 3
   One who supervises a person performing an activity, in order to help them should they be unable to complete it.
4. 4
   A person who gives directions for a crane or vehicle (that is challenging to navigate) from a different point of view to the moved load.
5. 5
   Dolphins of the genus Stenella, and specifically Stenella attenuata distinguished from the streaker (Stenella coeruleoalba) and the spinner (Stenella longirostris).
6. 6
   A team member responsible for communication with the driver to keep them up-to-date about on-track events, such as other nearby cars.
7. 7
   A person employed to assist a sports presenter by identifying the players or events during a game.