List full text indexes in Oracle database
There are 4 types of Text indexes in Oracle, owned by the CTXSYS user (installed optionally), so you can simply query [DBA|ALL|USER]_INDEXES and filter rows by the ITYP_OWNER and ITYP_NAME columns:

select
  *
from
  dba_indexes
where
  ityp_owner = 'CTXSYS'
  and ityp_name in ('CONTEXT', 'CTXCAT', 'CTXRULE', 'CTXXPATH')
;


Index types can be found in the DBA_INDEXTYPES view.