Flirt with her first, see how she takes to it? Talk to her friends, see if she's into anyone? Girls talk about everything so she will find out you've been asking about her and she'll definitely know you're interested. Or ask her for help with a shopping trip or something, so it's not a definite date but you'll be alone and can judge how she feels about you?
Any good relationship must have honesty, respect, caring, and a willingness to talk about anything. If she is already your friend, then ask her out. What are you waiting for?
All she can say is "no, thank you", and you both keep on going. Go for it.