Some of the best relationships I've ever been in began as friendships adn escalated from there. I agree with YourEternalRest; she'll start picking up on it. Girls generally know when we're interested before we do.
Just start spending more time together, especially one on one. She'll get the message. Put your hand on her shoulder or offer her your arm, and see how she responds. If she relaxes into it or brightens up, you'll know she is interested. If she stiffens or pulls away, then you're locked in the friend zone.
I also think Xero's suggestion has merit. "Have you ever thought about us being more than friends?" can be a good way to proceed. That gives her the opportunity to let you know that the relationship is strictly friendship OR to let you know that she's interested in more. Some girls do better with a direct approach, and some it scares them off. This is where you just have to read the signals and know the individual girl.
Best wishes. It sounds like it's definitely worth pursuing. Being friends with a girl doesn't limit the development of a romantic relationship. However, if the romance doesn't work out then you risk losing the friendship. Just asking won't end the friendship, but if it escalates to romance and doesn't work, then it's very hard, although not impossible, to preserve the friendship. That's just something you have to accept. I think it's probably worth it in this case, though.