Retrieving the unused materialized views

The following query will list the unused materialized views:

DISTINCT owner, object_type, object_name
FROM dba_objects
WHERE object_type LIKE 'MA%'
AND (owner, object_name) NOT IN
   (SELECT DISTINCT object_OWNER, object_name
    FROM dba_hist_sql_plan
    WHERE object_type LIKE 'MA%');

The object type for materialized views in the dba_hist_sql_plan can either be MAT_VIEW or MAT_VIEW_REWRITE. 

Please be aware that using the AWR views requires the appropriate license.