ALTER INDEX index_name_here MONITORING USAGE;
And after a while you can browse USED column of the V$OBJECT_USAGE view.
It tels is your undex been used. But it does not tell if the index is used wisely. ASH to the rescue. Possibly increase the time ash is stored. And here is a query that shows the clauses that have been using the index.
TO_CHAR(substr(sql_text, 1, 4000)) txt from dba_hist_sqltext where sql_id in ( select sql_id from dba_hist_sql_plan where object_name = :index_name) order by txt ;
If no index usage is noticed. The index is a possible candidate for dropping away. The case in V$OBJECT_USAGE.
But it might be so that the index is used in a non wisely manner. From there on you can interpret the plans. It might be that the full table scan would perform better.
And going to 12c there becomes the adaptivity of a cursor execution that will end up in the plan even it executes better in other execution paths. This was covered yesterday quite nicely by Nicolas Jardot. So for 12c the is the index used query here should be attached to dba_hist_active_sess_history. Future work to be done.
Before dropping consider making the index invisible for awhile.