I'd agree with the advice given already. We were subjected to varying degrees of polished performances by headteachers and pupils. All very slick and impressive but so are brochures and presentations for new housing developments. Until you "live" the experience for a bit, you have no idea how representative it is.
We learned to go "off message" and wander about on our own to speak to students away from the set tours and talks, but more importantly, we took the opportunities to see the schools during their normal working days. Have a nose into classrooms, see what the level of attention is like at the back of the class. How are students moving between classes at the bell, etc. Punctually? Reasonably orderly? It tells you a lot about the atmosphere and the control of the "hygiene factors". If the school has some of these smaller basics in place, it gives you confidence. As Guest55 suggests, check for general tidiness/cleanliness (the school and the pupils...!). If the indicators of these hygiene factors aren't great, can you be confident that the school is on top of progress, attendance, discipline, timekeeping, etc.?