How do I make a script that will function if a player LOOKS at something?

Let's say I'm making a Slenderman game, I don't want to make Slenderman touching them and then they're dead, I want the player to be actually looking at Slenderman to die. How would I do this like in that Doctor Who game with the Weeping Angels?

Well in Don't Blink it was much harder to detect if the player was actually "seeing" the angel, because of shadows and stuff, but normally creating a ray out of the player's head should be sufficient.