Just say hey sometimes when you see her and start a conversation, and when the conversation is good you end it and leave. She will be more interested after that. Next time you meet her ask her out.
The reason why she will be more interested is that you show power to her when you are starting the conversation and even more power when you are also ending it. ( Not needy)
She wants more, but can not have it until next time you talk to her.
If you want to now if she is interested in you, you must observe how she talks to other guys. If she are acting much different around you than other guys in a positive ore freaky( trying to be cool) way she probably have interest in you.