How to check primary and standby database are in sync in oracle?

We can check primary and standby database data synchronization just by running the following scripts on both database environments and the SEQUENCE# should be the same on both databases.

Example:

On primary database:

SQL> select thread#,sequence#, applied from v$archived_log
where thread#=1 and applied='YES' and sequence#<21144
order by sequence# desc
FETCH FIRST 2 ROWS ONLY;  2    3    4
    THREAD#  SEQUENCE# APPLIED
---------- ---------- ---------
         1       2804 YES
         1       2804 YES
 SQL>

And on standby database:

SQL> select thread#,sequence#, applied from v$archived_log
where thread#=1 and applied='YES' and sequence#<21144
order by sequence# desc
FETCH FIRST 2 ROWS ONLY;  2    3    4
    THREAD#  SEQUENCE# APPLIED
---------- ---------- ---------
         1       2804 YES
         1       2803 YES
 SQL>