npc_viewcone

npc_viewcone
is a console command available in all Source games.

The npc_viewcone of this npc_zombie makes it clear that the player is currently outside its Field of View.
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).

This Console command requires sv_cheats
set to 1
and then developer
set to 1
.