How to check the available free space in mongodb
Checking free space in the internal storage files of mongo is not something you normally (or at all) do.
Mongo provides a number of functions which you can use to check the size of collections and some helpful scripts demonstrating how to run the functions on each database on the server.
You can also wrap such functions in a script to make reading easier, such as this script which lists the size and counts for all collections in a database.