Monitor the UNDO tablespace extents

Below query will do it:

select status, count(*) Num_Extents, sum(blocks) Num_Blocks, round((sum(bytes)/1024/1024),2) MB from dba_undo_extents 
group by status order by status;

STATUS    NUM_EXTENTS NUM_BLOCKS         MB
--------- ----------- ---------- ----------
ACTIVE          16069   11519120   89993.13
EXPIRED           696       6048      47.25

UNEXPIRED        2286      21408     167.25