Script: Monitor the UNDO progress

select ses.username,
  substr(ses.program, 1, 19) command,
  tra.used_ublk
from v$session ses,
     v$transaction tra
where ses.saddr = tra.ses_addr;

Reference: http://gavinsoorma.com/2009/06/how-to-monitor-the-progress-of-an-undo-operation/