How to not get friend-zoned by her?

Well, I'm practically already friend-zoned. We work together, she give me rides home when we close together, I help her out at work sometimes, etc. We're comfortably getting to know each other, but not because were dating. She also got out of a ser. rel. a few months ago. Because of that and working together I keep my distance and keep the flirting minimal. I'm taking it slow. I want her to feel comfortable and know I'm not trying to get in her pants but do want more than to be friends. I want to be able to play my cards right and know WHAT risks are worth taking with her and not screw this up. She doesn't respond too well to overt flirting either, she keeps her cool which makes it hard to tell if she likes it/me or not. We do get along and make each other laugh often. I want to ask her out soon but she is just not giving me enough feedback and I don't want to wait long enough to be finalized as a friend. Any kind of advice/perspective would be greatly appreciated.