The best way to confront it without jeopardizing the friendship is probably to just tell her straight up. You don't have to break down and fess up or anything, but if you guys are as close as you claim to be, you should be able to catch her aside at some moment and tell her that you're interested in her as more than a friend...however you choose to do that.
The fact of the matter is, beating around the bush with the matter is probably gonna end one of two ways.
1) She won't ever catch the drift, will end up with some other guy, and your point will be moot.
2) Things will progress without ever really being defined, and if she isn't feeling the same way, then you'll probably end up finding out the hard way
You could just let the chips fall as they may and see where it goes, but that seems just as likely to miss as hit. Sometimes, you gotta put yourself out on a limb, and if your friend has her head straight, then I'm sure she'll at least be glad you were straight-forward about it. And at least you'll know where things stand.