I'll agree with previous suggestions, and also add that as a preschool teacher myself, I believe strongly in a play-based environment. This means that while there are 'invitations' for literacy, numeracy, science play, etc., that the kids aren't being drilled for comprehension, but rather, are allowed to discover the answers on their own, with support from the teacher.
How might this look? In my classroom, we have 'mailboxes' for each child, so the children can leave pictures and messages (sometimes they dictate it to me, and through watching my writing their words, they are interested and motivated to practice writing letters when they are ready). We have jobs each day which may ask a child to "count how many people are eating snack today" and then to gather that amount of napkins or placemats and set them out, which gives counting a purpose, instead of abstract rote number recitation (which is fine, but they need a purpose for numbers to understand what they mean). I would want to see teachers sitting close to the children and gently adding to the play by noticing things: "Trevor, you used all square blocks to make your tower, and Suzie is using the rectangles" without *directing* their play too much. Closeness and focus is also appropriate for providing social support to the children, because young children need a lot of coaching/scripting when it comes to conflict resolution/taking turns, etc. The biggest things children learn at preschool is how to get along in the group community, how to follow them through the daily routines and rhythms, and how to trust teachers and take direction from them.
Are teachers patient with the children as they work to get dressed for outside play (and there should be at least 30 minutes of outside play for every 4 hours attended, IMO), and are children encouraged to dress themselves to the best of their ability and then helped, or are the teachers impatiently doing it all for the kids? This is important to me, because this is something that slows Kindergarten teachers down in their teaching day. Self-help skills should be encouraged, so ask what happens when kids have accidents (they should be asked to help change themselves, but not to clean up urine or feces).
I personally also want to know what their discipline styles are, and if they are reliant on punishments/rewards to correct behavior, or if they are more intuitive, giving upset children space to play alone until they are ready to come back into the group, or a quiet chair to sit in if uncooperative, but out of the flow of the activity, so that the child can recover themselves and not be humiliated? Me personally, I've found out that too much reliance on time-out can create a poor classroom dynamic with more repeat offences than a more "working with" approach.
Are the teachers happy to see the kids, and do you get the sense that they like what they do? Important!
A more specific question: what sorts of toys/props are they offering? A lot of places have non-functioning noise toys, or toys that aren't well-cared for/broken, or 'character toys' which can really impede imaginative play. Toys ideally should be multi-purpose, so that the child can use them for different ideas/stories and in good condition. Likewise, the library area: books should be clean, without rips or writing, and hopefully not media-oriented. How a school keeps their classroom says a lot about how they esteem the child.
Overall, you are hopefully going to find a play that feels engaging, warm and inviting for your family. The best way to tell this is to ask to observe the classrooms in action. Happy, busy tones should be more present than the sad/upset ones. Teachers will speak respectfully to the children, and not make them the butt of jokes/sarcasm. Music might be on, but it's not the local radio station blaring over the kids, which can be disturbing and rattle little senses; instead, a more mellow cd is on. Adult conversation is appropriate to the setting and doesn't trump the child's needs for attention.
Kids who aren't quite ready to join the bigger group activities (say, sharing at circle/gathering times, or singing all the songs/fingerplays) are encouraged without a lot of fuss, and allowed to come up with answers or ideas on their own instead of teacher taking over. In short, that the kids have the environment which allows them to take chances intellectually and socially and that they receive the empathy and support young children need from their teachers.
I know this was long. Use your intuition and you'll likely be fine.