Describe the job in detail.
Agree pay, time off (if she has weekends free, say so, because au pairs are so exploited that she cant take that for granted).
Tell her about her living accommodation (own room?), availability of English classes, health insurance.
Have a general chat about her life.
Make up your mind about whether you can work with her or not. Anything else is a waste of time.
Realistically, you can't check references that matter (eg in her home country), and there are as many bad families as bad au pairs, anyway.
My approach is to go on my instincts, and then keep a closer eye on her than she realises, when she arrives. Eg, chat a lot with the children to find out what she is like when you arent there (dont question them directly, obviously). Also, invite her to feel free to use the computer whenever she wants, and then check up what websites she has visited, how long she has spent on it etc when she is not around.
If she is a reasonably normal person, and you don't ask her to do loads of housework, you should be OK. It is a lottery, but I dont think a rigorous selection routine shortens the odds.
Our biggest au pair disaster was a very nice girl, with a completely batty mother. Im not ready to relate that story on MN yet!