Hey I`m in the same position dude !!
Get a bit adventurous, and pay attention to her responses.
See if she comes for long night walks with you, if she remembers every little thing you said. Stand a bit close to her, check if she takes up defensive posture or she`s comfortable with it. Like italianboots said, stare into her eyes and watch her response. Say something like "If only you were single......".
Think of more such things but try them only when you both are alone !!!
She maybe thinking of having two guys at the same time but if she is really your good friend she won`t do it.