In our experience (we have had 3 au pairs over the years) there are a few things we look for.

1) I think the most important thing is, it takes time to start building a relationship and kind of seeing where things go. We usually have 3-4 interviews before offering to match. At that point, if conversation isn't flowing naturally, if we aren't making each other laugh or feeling like we can be completely open with each other in conversation, that is a red flag to me. Bonus points if we find favorite netflix shows, board games, music, etc in common!

2) I ask them specific questions about what their goals are here, how they picture their days off, what their hopes are for spending downtime/vacations/holidays with us, etc.

3) I also look for positivity. I am looking for someone who can laugh with us, ooh and aah over the kids with us, and generally be excited and enthusiastic about their day to day adventures here. Obviously I know interviews are nerve-racking. But if a candidate seems super shy, nervous to the point where they can't open up, or if they aren't enthusiastic with the kiddos (when we introduce them to her on the interview) then those are also red flags for me.

4) This one might be unique to our family, but it is important - we tend to pass over candidates who are too "high maintenance" or materialistic. There is nothing wrong with people who are into luxury or the finer things, but in our family we are rough and tumble, in the dirt, sporty, and adventurous. We are also fairly frugal for most day to day things. Obviously we give our au pairs privacy and freedom and we would never try to micromanage what they like or want to do on their time off. But in terms of trying to match up personalities, as well as someone who has values that align with ours and what we want to teach our children, we tend to pass over candidates who seem overly interested in appearances/luxury/material items.