How do I get this girl to like me back?!?

I have a really close friend that I talk to all the time. She is so perfect and I'm truelly falling for her. I just flirt with her every once in a while and I have yet to just go for it and ask her out. I really don't want to lose her as a friend so when I ask her out I need to be certain she likes me. What can I do to get her to like me back while still being discreet??