Viewing a school tonight

At dd's school all year 8 and 9s are expected to be guides at the open evening. Parents have to write to the head of year if they aren't able to, so they don't handpick kids they think will create a good impression

When dd2 and i were doing a tour of dd1's school at the open eve i heard a parent say to a teacher that their (pupil) guide was being VERY honest. Would love to know what he was saying!

Talk to the kids!
Other thing is what the school looks like to a certain extent. You could see that a local secondary school was struggling because of the way it looked. It was less the tired buildings (get lack of money) but more it didn't seem to care how dirty it was (chewing gum on carpets, loo roll across the floor etc)

Walking round the school will give you an idea of how big the school is, how it organized, how the space is used, what resources are available and an idea of the atmosphere.

Other than that you need to go in with focused questions about academic/pastoral/extra curricular activities depending on what you are interested in.

If you're shown around by a student, they are also pretty good on "telling it like it is!"

I would not be fooled by the talk by the head teacher/ principal. You need to assume it will be a slick presentation presenting the school in the very best light. If it is not then that to me is a small red flag as to how good the school senior management is and how little emphasis they are putting on the presentation. Are they just super arrogant they will have a full school in the future
If the head / principal is making comments along the line you must put this school down as first preference or you will not get in, then that is a big red flag. Firstly because that is just not correct, you can get in, the school being your 2nd or lower preference and secondly for the shear arrogance of what they are saying.
I would be much more impressed if pupils at the school are showing people around. That says to me that the school have the confidence in their pupils to be their spokesperson.
I am a governor at a large secondary school which is over-subscribed because of parent's views on the excellence of the pastoral support and I cannot under-estimate the importance of this when things go wrong for a pupil at any school.
We have recently just moved into a new building and a concern for me is the number of parents that are basing decisions on a swish new building when it is what is behind the new facilities that matters.