Tw1nkle
Visiting school open days is getting trickier - these are becoming very slick PR affairs where the people chosen to be there who are non-staff are hand-picked and the staff all sing the party line.
My advice is ask the questions but also use your eyes and common sense.
I'm sure you've looked at the latest OFSTED report for the schools you are considering - if not you should. If there is a letter that they'll be inspected by OFSTED soon, look at the last report and consider if they've had a new head since then.
Look at DofE KS2 SAT results - but over the last 5 years - what you're looking for is consistency. My opinion is far better to send kids to school that consistently gets >85% KS2 reading/ maths/ science L4 than a school that gets 100% one year at 67% the next (like ours) - if a school is all over the shop for a run of six years - that could suggest underlying problems (not just the odd bad year of pupils).
My top 3 questions would be.
If your concern is before/ after school clubs so that school can fit around your work - make sure you do ask this. 3 schools in the area for us but our school (which we're not thrilled with, but is by no means awful) does have great before and after school care (which we are thrilled with & makes live very manageable).
Find out what the average amount of parental contribution to field trips will be per year. If you don't want to be forking out >£200 a year - for example, you need to understand which schools always have residentials and what your child will do if they don't go (because you don't want to pay or can't pay).
Find out where children from this primary usually go to secondary.
After the talk parents are usually invited to walk around the school and have a peak (sometimes this is unguided - sometimes with pupils leading you around).
Really talk to the students. (Now be aware the school will have carefully chosen these students - but really ask them about their experience of school). If they're hesitant - say you ask is there a lot of bullying in the play ground - and they really hesitate and don't really answer - that's a signal there may be problems.
Often schools will let you know there is a forthcoming summer fair - go along to this and talk to parents. For a few pounds - you may get a chance to find out from actual Mums and Dads that it is a chaotic place, but fantastically happy and the kids seem to thrive or that although orderly and neat, the teacher's are dreadful and belittle or threaten the children all the time.
Talk to neighbours - talk to parents walking their kids to school in your area.
Finally - just accept that no school is perfect. They'll be bits you think are fantastic and bits that worry you or you just hate. There is bound to be a year with a teacher who isn't your cup of tea. The point is will your DC be happy there and will they thrive in that kind of environment.
If possible take the DC along as well - they pick up on things we can't possibly imagine. DD1 didn't like her 1/2 hour with the YR class at one school - turns out it had a real bullying problem. She just kept asking not to go to that school but at 4 couldn't explain why. We decided there must be something, so put it as our 3rd choice. Subsequently we've had several children transfer into our school from that school for reasons of bullying.
HTH