Librarians purchase books which have won major awards, including all Newbery, Caldecott, and Printz winners. We also purchase many of the Batchelder, Pura Belpre, Geissel, and Coretta Scott King books.
Both Newbery and Caldecott award winners have stickers on their spines so it's easy to identify them:
The Cooperative Children's Book Center at the University of Wisconsin Madison provides an exhaustive list of awards for children's literature.