Okay, you have two options.
Option One: Go For It
1. Just ask her out.
2. Make it romantic. Somewhere nice. If she accepts, she's definitely interested.
Option Two: Cautious Approach
1. Find a reason that you need to meet up. Subtle excuses include:
- Study group (you can't focus at home, does she want to join you in a free?)
- Lunch (the people you normally sit with are staying in over break, is it okay if you sit with her?)
- Can't find a specific room/location (does she know where student finance is?)
- Want help with a project (especially in photography or subjects where her opinion might be needed)
1A. For even more subtle options, ask a group of people to an event and include her in it. Repeat 1/1A as necessary.
2. If she accepts, continue to find reasons. Try not to repeat the same reason, unless as a study group. This gives you time to decide whether or not she likes you.
3. After an appropriate amount of time, refer to option one.