OP, how long have you had these feelings for this girl? You say you're normally smooth, so treat her like you would any other girl you're pursuing. It will be much easier to figure out her feelings because you shouldn't have a hard time getting this girl alone. I doubt you're as smooth as you think, or you'd just make a move on her. Don't worry about ruining the friendship or whatever. Next time you're out with her, go for a kiss, and see what happens. If she likes it, great. If she doesn't, then decide whether to cut her off or continue to be an orbiter of hers. I suggest cutting it off, at least for a while.
InnocentMan is right. You really do just need to get this bird pumped at this point. Make a deft move, and move on if she doesn't go for it. Whatever you do, don't have a conversation about feelings with her, unless she is the one that brings it up.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh