Hair if its a boy.
and the face if is a girl

Face or there weight some times.

from a distance i firstly look at the way the person walks.. gettting closer i take a look at face and figure, then at moves they do (gestures and other stuff)

I look at her face and concentrate on her chest

If it's a boy, legs <.<

If it's a girl, Arms haha

first i look at the general shape of the person... weight hair skin color height etc..... then i compare this shape to others that have similiar shapes becuase i've noticed that people who look similair tend to act similiarly

then i look at clothes. skate shoes most likely a guy i'd rather not talk to... but ocasionally will based on other apearences and behaviours around others.....pants if they sag id prefer not to talk to them... wear a belt jeez.... to much random acceserries ex. rings necklaces etc if its a guy other wise its okay..... eyes see what kinda soul this person has.....if the guy squeeks when he talks... if the person is calm or angry.... how other people look at this person.....
whether they fidget with random objects etc etc....
list goes on.... but those are the basics

You mean your face would be facing her face
and your eyes focusing on her chest?

That would never work.
It'd be way to obvious.

^ not if your wearing shades, and six foor four like me...

If it's a guy
~Build
~ArmLength
~Hair
~Eye's (if they hold your stare/ glance, they go up in my books...

If it's a girl(sunnies)
~Weight
~chest
~Head/face
~(passing by) side boobs, general side structure
~(back) Nothing keep on walking without turning head

if it's a girl (no sunnies)
~Weight
~Walk
~eye's, chicks love a guy that can look them in the eyes

Build, face, clothing in general.

I make a quick assessment of what kind of person I'm dealing with, and act accordingly.
Not going to start about certain subjects (science comes to mind) when having a conversation with someone that's muscular/fashionable/a girl :v

lol'ing at all people in this thread going 'i look at if theyre body is hot and their tits.'
One doesn't look at breasts, one subtly sneaks a peak from the bottom of the eye while having a conversation.

Conversations are too long. Most times your having a conversation with a girl you dont know them directly, it's either a friend or family that is introducing them to you. Thats worse if you get caught having a look at their tits...

walking by is the best chance to "sneek a peek"