How to check Data Guard transport and apply lag in Oracle

V$DATAGUARD_STATS reports the two lag numbers that matter for RPO: transport lag (redo not yet shipped) and apply lag (redo shipped but not yet applied). Filter to %lag to get just the interesting rows.

SET LINESIZE 200
COL value FORMAT a30

SELECT name, value, time_computed, datum_time
FROM   v$dataguard_stats
WHERE  name LIKE '%lag';