Face. At my worst, I'm just making sure she won't notice me checking her out. When I examine a face, I notice how she holds her jaw, how observant her eyes are, and how she holds her head. I'm assuming we aren't making eye contact.
After that, I observe posture and clothing. You can tell a lot about a person by those two. Posture and stride are very telling. With clothes, it's more than what brand or how much skin is displayed. Color choice, quality (ironed, clean, worn out), how it rests on their body, style, everything gives you a hint of who they are.
If she's facing me, her face. Usually her lips since I have a hard time, even during a conversation to look people in the eye because I'm half deaf. Usually when I look people in the eye when I talk to them I might not hear everything, so I tend to read their lips when they talk just to make sure I get everything they say. But if the girls back is facing me. I'm going to be checking out her ass.
Face. 3 seconds are needed for us to either put you on the "I can have sex with her" - side or the "Let's hurry this up and end it before I say something way too stupid". Then I concentrate on eyes and how they look back at me. Hands and feet then, shows me pretty much her body type, breasts for a second or two and then if I like this girl - eyes only. If not - eye contact is weak and I don't really concentrate on a speciffic part of her. Or at her at all :)
Face or ass. I just do. They're the things I like the most.
with every girl it's different.. it all depends what stands out the most. If she has big butt then that would catch my eye first then I would look at everything else or if she has very wild eyes or really nice t*ts or if she has track legs I love those ;) and her jaw. If the girl has a sexy jaw and is slim I would find her insanely attractive.