go for the kill dude! kidding...but seriously, just ask her, you know as you would normally. no need for anything fancy. relax(i know how hard that can be) go up and introduce yourself.
do not make the same mistake i once did: IE wait too long to do anything, and in the end she ends up with someone else. the perfect time never does come. so have some confidence in yourself! You dont want to spend time in the future wondering why you didn't do anything, or what could have been. take action and dont regret it. life is too short, seize the day. Just make sure you dont do it in like a creepy way like stalking of cos. girls hate that.
also, im assuming you dont know her. if you happen to be her friend, that may be a bit tricky. in that case, best to know a bit more(any mutual friends you can use to gather information?) on whether she'll likely want to go further with you. the prob with a friend is that if she does reject you, it's gonna be difficult to be friends in the future and the question becomes are you willing to risk the friendship for love instead-it becomes a bit of a gamble. im not saying it's not possible to continue being friends after that, heck i've managed to build back a frenship from such an akward situation, but suffice to say it'll be difficult to do so, and even at best, both of u will be in a very akward situation for at least 2 weeks- 2 months.
Anyways, do keep us updated on your situation and i wish you all the best.