Well, that depends on how well you know the girl. If you aren't sure whether she's interested in you, you could try something like this - you could begin by asking if she's doing anything special tonight. This would give you some indication of how interested she is. For example, if she says 'I'm going out with my boyfriend', then you'd maybe give up! If she says she's working, or visiting her grandma, then maybe you have a chance, and you could ask her again another time. Or if she says that she's doing nothing special, then perhaps you could say something like 'Do you fancy going for a pizza? I've heard that the new Italian restaurant on town is pretty good?'.