You need to build enough comfort where you don't have to ask. Assess her INTEREST, and go from there. If she is returning your kino, eye contact, laughing at your innuendo's and contributing her own, she is interested. Why would you ask a woman to go do something with you if she is not comfortable holding your hand, hugging you with her breasts tight against you, and initiating touch?
You asking her out on a date in person is just calling her actions to the forefront and if she doesn't match her actions then and there to going somewhere alone with you , then bail. Let your actions show her it is ok for her to show you she is interested.