Do you ever feel like someone's watching you? For eg, you get that "feeling" someone's eyes are on you and when you turn around, the person quickly looks away? You know there were science experiments conducted on this to prove that humans can "see" "know" and "sense" things without physical sight which defies logical explanation (when u think about it).
The experiments failed to prove we have eyes at the back of our heads coz it was really challenging to have proper controls. For instance, if you knew you were part of the experiment, your heart rate might be higher because you're waiting in anticipation etc so the results weren't accurate.
So in conclusion, science can't prove we know people are staring at us when they do but soooo many of us have experienced it so I say we have a sixth sense. Well, if I stare at YOUR back from across the room at a party and you turn around, I'll be thinking of this blog post! And I'll have one of those ah-ha! moments :)