Spotter

Spotter is a perk in Fallout: New Vegas given to you while you have Craig Boone as your companion.

This perk will highlight all hostile enemies in red while you are "actively aiming". This means using iron-sights or a scope. Enemies will glow green instead of red when using a night-vision scope. This makes enemies easier to spot and attack, especially at long distances or at night. This perk does not, however, highlight cloaked enemies, such as nightkin.