look into any complaints that have been filed against them. make sure their paperwork is in order (in other words, make sure they're legal), visit the site and look at the condition of the building and level of cleanliness. look at the children who are there. see what material they use for educational purposes. ask what their schedule is. find out the typical child:adult ratio. make sure they seem professional and organized. make it clear that you will be dropping in unannounced. that would about cover it for me i think.
see if they are licensed and review there past records. Thats a biggie and see what there ciriculum is like. How involved are the teachers with the children? Meal plan, rates, cleanliness teacher to child ratio. these are just a few that I can think of off hand. I work in an in home center and these are some of the most asked questions.