First, what I did was look at all the preschool pedagogy's. Waldorf, play based, Montessori, co-op, and even Headstart. After narrowing down which method I felt was best suited for my children (Montessori), I started touring Montessori school's in the area. I went to 5 or 6 schools and narrowed it down to 2. We went back and forth on our decision and finally picked one (luckily we didn't pick the other because it's now out of business after many years) based on:
*How the teachers treated the children (we do positive discipline, choices, and I wanted mutual respect with no time out's).
*How comfortable my child was with the teacher
*Was this an authentic Montessori school that uses correct Montessori materials and methods.
*How the children interacted with each other.
I also met another Mom here online who had a child at the school we picked and she gave me a lot of good info. I think you should definitely get some parent references. Maybe ask the school if they can provide that for you. Good luck!