Find some excuse to talk to her. If she's at all interested it will be obvious.
I leave it up to fate. Usually you can't ask an unknown woman for her number even if she seems friendly, so I leave it...if I see her again, and she's not with an obvious significant other, I find some way to flirt a little and if I'm feeling brave that day, pat her on her bottom.
9 times out of 10 she'll get angry or pretend to, but this is a situation where I wouldn't likely see her again anyway, so nothing to lose!
Look at her wistfully..and say "damn if only" in my head :)
If she looks at me and I think I might see something in her look or we happen to like be right near each other or have to talk or something I might ...flirt alittle or show some "like you maybe get to know each other" signs