In doing research for the methodology and curriculum I would be using for the preschool, I have read many descriptions of a high quality program. HighScope provides the following list of components:
Child development curriculum with active participatory learning
Low enrollment limits so adults can give individualized attention
Staff trained in early childhood development so they can observe, understand and support children’s learning in all areas
Supervisory support and in-service training so staff can understand and carry out the curriculum
Involvement of parents as partners because they are the children’s first and most lasting teachers
Sensitivity to the noneducational needs of children because preschool is just one component of children’s early experiences
Developmentally appropriate practice and evaluation procedures to accurately measure what children know and to plan effective ways to extend their learning.
I thought this was a great comprehensive list to start with when choosing a preschool for your child. There are definitely more detailed and specific questions to ask and things to look for in a preschool which I will post about later.