@boomerang,
Not sure if this is the best advice or not, but some things I asked:
Teacher/student ratio
Handling of discpline (this was more because I was afraid they might be too strict - paddling for instance as I found one school that did paddle)
Ask about teaching styles/how they teach/what they teach - curriculm/classes/class structure - it is important that this fits with how best your child learns - structured was good for my girls, but not for all kids
I am assuming you will meet some of the teachers - take the time to talk with them - you should get a feel for how they are with the kids
Other activities outside normal class stuff - art/music/team sports/
Do they do anything like spelling bees/math or art competitions if this would be an interest to your child - I like that my kids' school does this - it emphasizes academic competition as well as athletic competition.
Think about what is important for you about a school and ask those questions.