As a mom, I know how nervewracking this situation is...wondering whether or not you've chosen the best preschool for your child. They're too young to clearly articulate their experience so it's hard to really tell! I ended up enrolling my son in three different preschools before we found the "right" one for him and our family.
Even if your child may not be able to tell you with words how she is doing at school, you can probably tell by her behavior. (If she gets very upset at drop off, if you notice a big personality difference, if she's not sleeping well, she may not be happy where she is). But I think the best way to decide is to spend time in the classroom. Make arrangements to read stories to the class or do a project with the class - or even just have lunch or snack there. While you're there, pay attention to how the teachers work with all the kids (including yours), how the classroom is set up, how happy the other kids seem, etc.
If you do this a few times you'll have a really good sense of whether or not you made the right choice.
The articles I'm posting below may also help. The first is a list of 10 things to look for in a great preschool. The second is an article about how to choose the right preschool (although I know you're already past that point).