Look for happy children who are respectful towards one another. Watch the teacher and any assistant during conflict resolution. Make sure there is consistency within the teacher's and assistants resolution styles. This is so important. It's the same as a healthy family away from school.
School is such a giant step.
It is important to make a conscious decision on your child's education based on fact finding through observation and relying on your gut instincts. One of the things I would initially look for is student-teacher interaction. This can be accomplished through several mediums. First, I would schedule an interview within the classroom with the child, parents and the teacher. Watch how your child interacts through the environment.