Check the undo tablespace usage

col NAME format a15
col TABLESPACE_NAME format a10
col Programme format a20
col machine format a30

SELECT
a.usn,
e.sid,
e.serial#,
e.username,
e.program,
e.machine,
e.osuser,
a.name,
b.status,
c.tablespace_name,
d.addr
FROM
v$rollname a,
v$rollstat b,
dba_rollback_segs c,
v$transaction d,
v$session e
WHERE
a.usn=b.usn AND
a.name=c.segment_name AND
a.usn=d.xidusn AND
d.addr=e.taddr;