I guess most of us have some hidden talent to notice from far distance about your crush .You may noticed from the color of their dress, body language, hairstyle or anything that might have been save in your memory that your crush used to have.
So when you see one things which belongs to your crush which is in your memory there will be some hormonal surge that activate the sympathetic nervous system; this is the same part of nervous system activated when you are being chased by a wild animals, or when you just safe hitting by your car to baby passed nearby . The effects of this are what someone describes as an adrenaline rush. There will be increase in your heart rate, some might experience increase in breathing rate, dryness in your mouth, sweating and so on. So, i guess many of you who are reading this article might experience above features.
Activation of sympathetic nervous system causes pupil dilation (which allows us to take more visual stimuli), decreases the curve ness of the lens (lens become less convex), these changes are normal when we try to see far objects. So this might give you some inborn talent to view someone far more clearly especially your crush or something which is about your interest.
Note: There is Parasympathetic nervous system also that is activated when you are in rest.