I ask if many children leave the school and why
and if many join later on.
some schools have this a lot due to geography, but my experience is that I like schools which attracts children who aren't thriving in other schools; can be due to a tolerant, forward thinking ethos.
I always ask the head 'how would you describe the school', of course it's an impossible q, but you can get a lot from what they choose to say, again, I like it when the answer is something like 'we think every child has something to offer', less keen on, 'discipline's really important here', but those are my priorities, won't be the same for everyone.
Do the children seem happy, do you hear laughter?
How are the children with staff, inc. head? Anyone can be sweet to kids when someone's watching; if the children seem relaxed and happy, not nervous with the adults, that's a good sign that they're usually like that.