@fireface,
fireface, first of all the best advice is first completely ignore anything anonymously99 has to say. That advice will get you no where. Secondly you already know what you need to do, so it is a little silly asking what you should do.
It might already be too late really. If she is aware of you and all this time you have said nothing or attempted to make your relationship with her stronger then she is going to wonder why you waited so long.
It is similar to finding a job. You need to make a good first impression. Not linger around waiting to get up the nerve to ask for a job. I know people will hate this analogy but they hate it because it's true. The same response for a job interview happens with dating. The person doesn't care if you are qualified for the job, they want to see what your personality is like. If you have a terrible personality, lack confidence and can't make up your mind or make choices, chances are you will be a crappy employee.
It doesn't matter if you have dated someone in the past. It is all about who you are now and what type of person you are. If you wait then it shows you lack any confidence. It is not attractive at all. The fact that you came here to ask what you do means you lack the ability to be yourself spontaneously to what ever happens if you approach her. You want a dead locked step by step process and relationships don't work that way.
The fact that you know she might be moving away already says you know too much about her at this point to all of a sudden want to start something with her. Unless she has absolutely no relationship prospects you really have no chance to really make a good impression at this point.
My over all advice is, if you are attracted to someone and you really want to get to know them or possibly see them as a potential romance, you shouldn't hesitate at all. Jump right in and get to know them, ask them out, flirt, make them laugh and show interest. It is better than looking down at your feet, pushing your toes into the ground and cramming your hands into your pockets while your face turns red as she's asking you what you want.