The Google Licensing Manager API, https://developers.google.com/google-apps/licensing/, can be used to assign Drive licenses to users in an Apps domain. How can you retrieve the amount of storage a given user is actually using and not just what their total quota is?
This can't be done in the Licensing Manager API, it only retrieves information about the actual storage licences. The amount of quota a user has actually used can be found with about call in the Drive API, documented at https://developers.google.com/drive/v2/reference/about/get. Specifically the quotaBytesUsed property.