How to check for active usage of an UNDO tablespace

SELECT a.name,n.status
FROM   v$rollname a inner join v$rollstat n
USING(usn)
WHERE a.name IN (
                  SELECT segment_name
                  FROM dba_segments
                  WHERE tablespace_name = 'UNDOTBS1'
)
;

NAME       STATUS
---------- ---------------
_SYSSMU8$  PENDING OFFLINE

Status can be either ONLINE, OFFLINE, PENDING OFFLINE or FULL.

* ONLINE means that the undo segments in the UNDO tablespace is being actively used
* OFFLINE means that the undo segments are not accessible for usage
* FULL means that the tablespace in which the undo segments reside has reached its limit, and you will most likely have seen the error

ORA-30036: unable to extend segment by  in undo tablespace 'UNDOTBS1'

at this point

* PENDING OFFLINE means there are still uncommitted transactions depending on the information in the undo segments in this particular UNDO tablespace.