How to check temp tablespace usage in Oracle

Temp tablespace totals from the space header, plus the sessions using temp right now so you can find a runaway sort or hash join.

-- Temp tablespace usage
SELECT th.tablespace_name,
       ROUND(SUM(tf.bytes)/1024/1024)      AS total_mb,
       ROUND(SUM(th.bytes_used)/1024/1024) AS used_mb,
       ROUND(SUM(th.bytes_free)/1024/1024) AS free_mb
FROM   v$temp_space_header th
       JOIN dba_temp_files tf ON tf.file_id = th.file_id
GROUP  BY th.tablespace_name
ORDER  BY th.tablespace_name;

-- Sessions using temp right now
SELECT s.username, s.sid, s.serial#, u.tablespace,
       ROUND(u.blocks * (SELECT value FROM v$parameter WHERE name = 'db_block_size') / 1024 / 1024) AS mb
FROM   v$sort_usage u
       JOIN v$session s ON s.saddr = u.session_addr
ORDER  BY u.blocks DESC;