Hm. (As you can't get a sense of anything beyond skin-deep just by checking someone out. ) I'd say her eyes.
I always find a girl's eyes her most attractive physical feature (regardless of how stunning or homely the rest of her may be), and catching that first flirtatious glance is a great deal of fun. I do find her face and general figure attractive, of course, but eyes top the list by far.
This may be because every fun, attractive girl I've known has pretty eyes. *Shrug* But there are old sayings eyes being windows to the soul, so perhaps there's some truth to it.