After not getting any of our 6 choices (living near London and schools having small catchment areas) for my asd son and going to various open days only for him to temporarily go to a giant feeder school the LA sent him to that was undergoing building work, had the senco on maternity leave and the head teacher die in his first year I took a different route for my daughter. I just made one choice and the one down our road.
You can't beat word of mouth, if you've got a few schools in mind see if there's a forum or website with reviews. Luckily for us the best school in our catchment area is the closest one to us so it was an easy decision
I agree that "knowing" too much can be a problem. I certainly saw through gimmicks and some of the things that were said when I viewed schools. In the end I went for the local school with a view that if I'm not happy with it, I can move him.