Assume you have a Snapshot Standby configuration: during nighttime, it is used as standby, during daytime as test / development and you have set up DB-Links from primary to standby and vice versa with the appropriate grants.
select d.database_role, dgs.* from v$dataguard_status@primary dgs, v$database@primary d
select d.database_role, dgs.* from v$dataguard_status@standby dgs, v$database@standby d
where message not like ‘RFS[%]: No standby redo logfiles created’
order by timestamp desc
is a great query to see simultaneously what happened one which node