You should look at the environment and how much room he has to move around. Are the areas crowded or will he have room to go into different areas in the classroom.
Ask about the daily schedule. How is the day structured. I've taught preschool and head start for 8 years and you need a schedule! It should have time for gross motor or physical activities. A time for free chice of learning centers and creative expressions (They should be doing art daily but no child should be forced to do it)and in preschool most should be doing group time. Group time usually consists of reading stories and having discussions. they should be challenging your preschooler to learn about new concepts and ideas. I would ask about a rest time. If your child still naps or even if they don't what is the classroom routine during quiet time.
A good thing to look at is also the art around the classroom. It should be displayed at the children's level and it should be process art not product art. By that I mean not just coloring sheets or pre-cut "pretty" art that looks perfect. The best art is the one that the child does unassisted. For example my daughter brought home a picture once of herself with her eyes on her stomach and shoes on her face. But she was proud of her work so I praised her for it. The art shouldn't all look the same because they should be doing it how they want to do it. If there is no art around I would wonder what they we're doing all day?
I would also ask about meals, are they provided or can you send a lunch. Do they post a menu clearly so you know what they are eating each day.
Also ask about fieldtrips and other learning experiences. Do they have music, computers, etc.
It's hard to pick a school but preschool is good way to help your child develop the social skills needed prior to entering kindergarden. My daughter loves school now and all her new friends!
Good Luck
J.