spotter (n.)

• beagle
• counterspy
• dick
• eye
• fireguard
• flatfoot
• gumshoe
• hawkshaw
• lookout
• operative
• patrol
• patroller
• patrolman
• picket
• roundsman
• scout
• sentinel
• sentry
• sleuthhound
• spy
• tec
• watch
• watcher
• watchman