To list all the indexes for a table:
select index_name from user_indexes where table_name = 'TABLE_NAME';
be aware that case is significant and that table names are in upper case.
If the table is a system table then it will not be listed in the user_indexes view, so instead use all_indexes:
user_indexes
all_indexes
select index_name from all_indexes where table_name = 'TABLE_NAME';