It's far better to just hang out with her a lot. Don't talk about having a relationship with her. Just have fun with her. Kiss her. Flirt with her. If she wants a relationship, she will bring it up. I assure you of that. Then, when she's the one that brings it up, you KNOW she has extremely high interest in you. There's nothing wrong with casual dating.
Plus if you are worried about some other guy "stealing" her away from you, i would encourage you to try to not worry about that. Think of it this way, the word "relationship" is really nothing more than a name. You can have a de facto relationship with her just by hanging out with her a bunch and her having high interest in you. If she has high interest in you, she probably won't go for other guys. And if she does, who cares! Because you know you are the more attractive man and that she's smart enough to choose you over all the other guys pursuing her.
Don't ask her. If you have discipline and patience she'll ask you.