If their looking at a girl with nothing but lust on their mind, they're gonna look at their butt or boobs. If they look at their face, their thinking about something more than lust.
Sometimes a guy will look at a girls face too much and not notice other features. For example there was this girl that I knew really well but only looked at her eyes, then one day I noticed her breasts and thought damn this girls busty (they were in front of my face).
I also find that girls that I will want to have a relationship with, I'll be looking at their face more than their body and will only notice their body if something attracts my attention to a certain part.
Yea we are pretty visual. If a guy is looking you up and down like that with a high level of enthusiasm he is probably thinking...DANM! There is a god...And for me it usually means I'm probably interested in you.
That's kind of weird, I've had girls I wouldn't want a relationship with ever, but I still look at their face cause it's what I find most attractive about them. Boobs are awesome too (not necessarily big, it's just whatever goes well with you), and then unlike most guys I know I really couldn't care less about a girls ass, it's just not something I notice.
In order of priority:
Face in general
...and by this point I'm thinking sunlight shines out of her ass and angels sing, so I automatically say everything else about her is perfect.