hmmm.... tough one....
but i would think if she treats you like a friend only, she'll talk
to you more about guys she like, and ask you about gals you
like....
also watch her eyes when you compliments other gals.... look for
any sign of disappointment...

that's all i can think of...
but seriously, if you like her... just ask her, or drop hints...
cos i've personally seen two compatible ppl NOT get together cos
they both do not think the other party sees them in that
way....
and if you don't like her in that way, likewise drop hints like....
"hmmm.... I bet if i had a little sister, she'll be just like
you!!"