How to give NPCs a Field Of View?

I'm currently making an AI, and I want to give it a field of view. For example; When a person pulls out a gun in-front the NPC, it preforms a certain action, Or when someone sneaks up behind the NPC, the NPC wont notice them. I've taken a look at the free Zombie AI provided by Roblox, but I am completely lost :P

Any help is appreciated!