Once all images on a tape have expired, there is nothing to report - all references to expiration dates have been removed and tapes are marked as unassigned / available.
In the Media GUI, the Assigned column will be blank.
You can use 'available_media' command in a cmd window and send output to a text file.
Anything marked as AVAILABLE has expired or else new (never written).
To monitor tapes that are about to expire, run the Media/Tape Summary report in the GUI on a regular basis (e.g. once a week). Be sure to select the 'verbose' option before you run the report.