npc_viewcone is a console command available in all games.
A console command that will display NPC fields of view. Two big red arrows will indicate the boundaries of the FOV (the horizontal boundaries of its viewcone), while a thinner yellow arrow indicates its Angle (where the NPC is facing).
This command takes an <npc_targetname> or an <npc_classname> as its argument. If no argument is specified, it picks the NPC that the player is looking at (through the crosshair !picker).