We are on our sixth Au pair - some are great - some are a nightmare to live with and make you hesitant to move around your own house for fear of what you might find!
From experience, I would say you need to ask as many questions as possible before they start. Its up to you to find yourself a good match and weed out girls who are prepared to lie to get the job - many do! Are their answers credible?
My current girl said she helped her mother around the house, but she has never lifted a finger before coming here. She struggles to use a tin opener, work the iron and even put a plastic sheet in the tumble dryer this morning. aaarghhh!
Most Au pair / family breakdowns are due to a mis-match of expectations, so if they know exactly what to expect from you and what you are really like - and vice versa, it is much more likely to work out.
Don't be afraid to spend a long time interviewing - and always do a second or third one, ideally on Skype so you can read their body language.
If you are house proud and keep a tidy household, a naturally untidy/dirty Au pair will struggle, be miserable and make you miserable (that's me at the moment!). Ask her how tidy her room is at home.
If good table manners are important to you, an Au pair that comes from a family that doesn't sit down to family meals or observe table etiquette may cause you undue stress (that's me at the moment!).
If your job involves more cleaning and less housework, and your au pair hates cleaning, same again....
Good luck! I could do with some...