There are resources available for doing research on preschools on-line but there are not as prolific as compared to the resources available for schools. Greatschools.org does provide preschool information for a good number of markets, which makes it the best place to start your search.
Once you do your on-line research, another great resource is other moms. I suggest you use your contacts or meet new ones. When you have little ones, two great organizations that offer opportunities to meet other moms with little ones are MOPs (Mother of Preschoolers) and the Mom’s Club. They provide a valuable service and resources. Sometimes they will even organize open houses for preschools. I have been a member of one of these organizations and it gave me the opportunity to meet some great friends and provided me with support and resources when I needed them most.