Thanks to everyone who responded.  I am only looking at the library collections and not the program as a whole.  I have put together the following criteria inspired by your responses.

Age of Collection:  How old is it? How many new books are added each year?

Condition:  What do the spines look like?  How much time and money is spent repairing books? When was it last weeded?

Breadth of Collection:  How big is the collection?  How many volumes?  Are there any eBooks?  Are there periodicals?  A professional collection? Enough to support the needs of the curriculum?

Depth of Collection:  How many sections do more than skim the surface?  Can this collection support multiple classroom projects?  

Quality of Collection:  Are the books from reputable publishers?  Are the texts age appropriate, attractive, and well-made?  Do non-fiction books come with a works cited or a webliography?

Award Winners:  Are the recent major children’s book award winners represented? (Caldecott, Newbery, etc)

Circulation Statistics:  How many book are checked out over the course of a month or a year?  How many books are lost or damaged beyond repair?