Hmm....i feel that most girls won't really express their liking
to a guy so openly or obviously, instead, they might drop some
hints like asking you out, showing concern for you everyday....thus
its actually quite difficult to know whether a girl likes you. But
of course, nothing is absolute in this world unless you're a GOD or
someone with exceptional powers. So i think the best is to ask at
the correct time when u can feel that you're close enough with her.
Unless that girl's character is somewhat on the "open" side, if not
i think girls will usually wait for a guy to take action first.