Oracle : How to check your undo segment

can't drop the undo then maybe one of its segment is
still being used issue this command to see the undo segments

SELECT RN.NAME,RS.STATUS
FROM V$ROLLNAME RN,V$ROLLSTAT RS
WHERE RN.NAME IN
(SELECT SEGMENT_NAME FROM DBA_SEGMENTS
WHERE TABLESPACE_NAME = 'NAME OF YOU UNDO')
AND RN.USN=RS.USN;