it's called the frum slide. it's what your eyes do when they latch on to a kippah or skirt at college.
first you affirm that it is indeed Jewish looking. that it is really a skirt and not just an incredibly baggy pair of pants. that it is truly a kippah and not a weird hat. then your eyes slide up or down to the face to see if it's a familiar one.
if it is, you rush over to each other, or maybe fall into step with them and do the whole catch up routine. ("what classes are you taking?" "who's your professor?")
if you don't know them, you do the shy -i-don't-really-know-you-but-you're-an-oasis-in-this-crazy-place-grin.