List users sorted by last logon date so you can see who still has *never* logged on, and who has not logged on in over x days or months.
Transaction se16 > usr02 table shows user and logon data. Field 'TRDAT' is the last logon date. Simply sort the results by this field, descending and you can see who has not logged recently. Perhaps they are candidates for deletion! A good time to do this is prior to submitting your USMM user audit.