I'm a big believer in going with your gut feeling. I had a similar experience with a day care provider, not preschool. I knew it wasn't a good fit for my daughter and she was not doing well there, but I felt bad switching her because I know change is difficult for her to adjust to. I spent two months anguishing about it. Then, thankfully, the day care provider gave her notice because she was moving. I had received a recommendation for a new one, and I went there with my daughter, took a tour, asked a lot of questions of the manager, and knew immediately it was the best place for her. Every day she tells me how much she loves her new day care. She never mentions missing the old one. I wish I hadn't waited two months to change, but I was chicken. The questions I would ask are:
1. Have you had students with SPD before?
2. How much of the school day are the children required to stay seated?
3. Is there a routine schedule of activities each day? Can I have a copy of one to see what a typical day looks like?
4. Which discipline method do you use?
5. (If your son has potty training issues) How open are you to alternative potty training methods? How do you handle accidents?
6. What's the teacher to student ratio?
7. (If your son is on a special diet) Do you have a problem with me bringing all of my child's snacks/food, even on special days?
All of these questions shed light on the school's flexibility, willingness to work with you, understanding of unique needs, and whether they're going to handle things so differently than you do at home that it would confuse your son.
All that being said, no preschool will be totally in sync with your ideas. My daughter attending a very good preschool, but I had two major problems that came up during her two years there. I had to swallow my cowardice and make a big stink about how they handled these two things. It was very stressful, but they did respond to my complaints and rectified the problems.