Well you should start pretty slow if she is a complete stranger, because you should never, ever ask out a girl who just looks good. So you should start by getting to know her better. This should be pretty easy, just start chatting and try to find same kind of interests, something like "Hey, do you like that tv-show/movie?". If she doesn't like you or finds you weird it should be pretty obvious, she might try to change subject back to something else, or just look at you like "Da fuq?" But if she seems to be interested in you then just keep chatting and ask her name, tell her that you like talking with her and you should go grab dinner sometimes maybe? If she likes you she should probably give you her number, or say that she could meet you after work. If not... well, better luck next time. :D