I suppose I go on gut feeling too. I quite like seeing the environment they Skype from as well as that gives a bit of info too, especially as my au pairs tend to be older.
I've done most of the checking before I interview, they have sample weekly schedules, have usually talked to our current au pair and I usually have a couple of questions initially as ice breakers, so something we both have in common. for example my current au pair likes harry potter and we have the studios nearby, so I asked what's her favourite HP book.
Previous au pair had mentioned that she loved our beach photos of the children so I asked about her favourite beach.
I like fairly confident (but not overbearing) intelligent women.
Before we Skype, I ask them to think of at least 2 questions that they'd like to ask.
So I check their reason for coming to the UK
I double check driving status etc.
I ask if they really like playing with children! I don't think our first au pair really did - although she was fab at housework.
I ask what they would do in an emergency, so for example if the house was on fire.
I ask what they would like to be doing in their free time.
If Englsih is not their first language, I check that they are happy to do a language course.
Then I reiterate that we want them to be happy here and ask if there's anything that they are unsure about. We don't do that much with our au pairs at the weekend and I check that they actually want that sort of placement.
If I think that they might be suitable and they are keen, I then bring in the children and let them say hello.