Index on a partitioned key
First, you can try to monitor index usage.
You will take the answer used index or not used.
Index monitoring can be switched on/off several times to get more information.

For example, here: https://oracle-base.com/articles/9i/performance-enhancements-9i#IndexMonitoring

Also, you can look into v$sql_plan view to see what recent queries used this index:

select s.sql_id, s.sql_text
  from v$sql_plan p, v$sql s
 where p.sql_id = s.sql_id
 and object_name = 'YOUR_INDEX_NAME_HERE' ;


Also, and most importantly, you can make the index invisible for the optimizer.
That way you can clearly see if it affects performance of your queries.

Examples - https://oracle-base.com/articles/11g/invisible-indexes-11gr1