Check with the licensing or accreditation group in your area and find out if the preschool is accredited and/or licensed. In some areas licensing is the only option for preschools, so research the possibilities. The state Education Department should have this information on their website.
Since most preschools are only half-day programs, is there an option for half day care for you child through the same facility or through a partnering arrangement with a day care. Find out about issues such as transportation if the children are moved to and from the preschool location.