Since this is a LOCALLY Managed tablespace , you can not check the DBA_FREE_SPACE for the un used space.

Well here we have some views :

select bytes_used,bytes_cached from v$temp_extent_pool;

another views to look at are
v$temp_space_header
v$temp_extent_map

V$sort_usage
v$sort_segment

You have to fight with the above view ...

Madhu