How would you have an object Detect that you're looking at it?

So that you can do actions to the object like grab it, toss it etc.
Like say a door knob.
if( Rift center of sight is looking stratight at me and user presses some button on the xbox controller)
       toggle lock or unlock door

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But i want only the closest object to be affected, if there are object in the line of sight but are behind other objects that are in the line of sight, they shouldn't react to anything.