DB Files in backup mode

To verify which datafiles are currently included in a tablespace that have been placed in backup mode:

SELECT t.name AS TB_NAME, d.file# as DFILEID, d.name AS DF_NAME, b.status
FROM V$DATAFILE d, V$TABLESPACE t, V$BACKUP b
WHERE d.TS#=t.TS#
AND b.FILE#=d.FILE#
AND b.STATUS='ACTIVE'