How does an object(nearest to player) detect that the headset viewing line is within its boundaries

Well i didn't really want cursors or UI. I want to interact with objects without having any cursors. Even people, lamps , lightswitches, computers, . And this one is an interesteing one i have planned. look at entire face of a person    VS looking at hand. Those two, after pressing an xbox button would give me some random results like the guy would get angy and say "what you looking at me for?"   or if i look at his hand, and press a key i would slap it(animation)