Just go up to her and make idle chit chat, not for too long and then say u'll see her around. you should get a better idea if she likes you if she flirts, doesnt look like she wants run away etc. Then next time you see her you can ask her if she wants to go out for a drink some time. If she says no, just say it was nice talking to you, see you around, that way you keep your dignity and if i was her would be impressed by the way you took the rejection. Good luck