The following steps provides a guideline to help DBA verify the function of the Dataguard after the physical standby database is restarted.

1. Make sure the listener is started before starting up the standby database
2. Startup the standby database in mount mode
3. Check the alert.log and drc .log of the primary database to see if there any errors
4. Check the alert.log and drc .log of the standby database
5. Check if the dataguard is working propperly.

On the standby:
SELECT SEQUENCE#, FIRST_TIME, NEXT_TIME FROM V$ARCHIVED_LOG ORDER BY SEQUENCE#;

On the primary:
ALTER SYSTEM ARCHIVE LOG CURRENT;

Then on the standby:
SELECT SEQUENCE#, FIRST_TIME, NEXT_TIME FROM V$ARCHIVED_LOG ORDER BY SEQUENCE#;

also verify new archived redo log files were applied:
SELECT SEQUENCE#,APPLIED FROM V$ARCHIVED_LOG ORDER BY SEQUENCE#;


Note that if you got ORA-16766: Redo Apply unexpectedly offline error in the standby database(drc .log) or archived log not being applied, thismeans you may need to start the redo apply manually.
ALTER DATABASE RECOVER MANAGED STANDBY DATABASE DISCONNECT FROM SESSION;