Assume nothing....
Be confident, if she says yes it does not matter....
If she says, no... its still doesn't matter
If you are worried about being in the friendship zone, its probably because you are for right now. Things to ask yourself, is does she do this playful kitten routine with other guys? How easy is it for her to get time from you to hang out? Does she make all the moves and I do nothing to signal back to her? Am I shy when I talk to her? All these questions have obvious answers, and you're the only one who can answer them.
Some will tell you that the friendship zone is the end of the road. I say not always, there are no rules in the game of love. Do not kid yourself this is a game in some ways, one we all like to play, otherwise their would be no humans to populate the earth. My best advice to you is to become a challenge:
Your time is valuable and you have a life you can't hang out with her all the time. Make her work to get your time and realize that you will not always be there. Let her see you openly and lavishly flirt with other girls, do what she does to you, expect with another girl. If you start dating, and get to a third date wait till a few days before and cancel it saying something has come up but you would like to reschedule. While she is sitting on her butt, watching tv and eating ben and jerry's she'll be thinking on the oppurtunites she missed out on earlier, and will jump at the next chance to be with you. It's worked for me and I'm just your average guy. Best of luck and protect your heart.
-Grey Fox