Index on which tablespace
The information is in the ALL_INDEXES (or USER_INDEXES) view:
select tablespace_namefrom all_indexeswhere owner = 'MYSCHEMA'and index_name = 'MYINDEX';
user_indexes.tablespace_name is null for partitioned indices.
Viewing Information About Partitioned Tables and Indexes suggests that user_ind_partitions view can be used to check containing tablespace for each separate index partition:
select index_name, partition_name, tablespace_name from user_ind_partitions where index_name = 'MYINDEX';