“Are you single” is the best way to convey interest.

If it’s a short engagement it gets straight to the point, gives a girl an easy out, and expresses interest. Ofcourse, it’s good to follow up with something interesting or complimentary. But even just “could I get your number to get together sometime?” Is fine.

Also, if you end up feeling chemistry during a conversation it’s easy and affective at getting to the point and again can have the same simple follow up line.