As an au-pair veteran, had au-pairs for the last 10 years and have finally given them up - till the DTs are school age. Still in contact with three of them.
I never, ever, take girls who have not been au-pairs before/not been au-pair in the UK. Always check refs with current/last family.
Not to say that those who have not been au-pairs before/in the UK before were always a problem - we had some great ones, but it always took much, much longer to settle in and show them the ropes - not used to the basics like the transport system, the traffic, the diversity and so on.
Often compounded by the fact of no friends - till they started going out and meeting people. Also more likely not to really understand/or want to understand what the role of an au-pair is - the agencies often sell it to them as being the elder sister in the house - and they think, great! In my house my mum does everything and it will be the same as an au-pair in the UK.
Asking a list of questions is meaningless - they say what they think you want to hear, even the good ones, afterall it is a job to them and they want to do their best to get it.
The only real way to know what you are taking on is to find out what they were like as an au-pair for another family - and you need to probe carefully "she is very homely" can mean she is lonely, homesick and never goes out.
Always surprised me how few UK families rang me to check au-pairs refs............