Hm... it's hard to show her that you like her in that way discretely, since you're already friends, but if I were you I would do simple things like hold her gaze a little longer, to make it a bit awkward, sit a bit closer... little things like that. If not, why don't you tell her exactly how you feel about her, and if she doesn't feel the same way I'm sure she'll understand -- this will probably have happened to her as well.
Hope it all works out!
