There's two great utilities for black-box analysis of databases. First is variety.js, which will give you a high-level overview. The second is schema.js, which will let you dig into the collections for more detail on the individual fields. When inheriting a production Mongo database, these two utilities can help you make sense of what's going on and how the collections and documents are structured.
mongo test --eval "var collection = 'users'" variety.js
mongo --shell schema.js