Manually comparing the data between source and target and optionally applying the differences
Please see http://docs.oracle.com/cd/B28359_01/appdev.111/b28419/d_comparison.htm for the use of DBMS_COMPARISON to compare both datasets and generate SQL for the differences.
Another option is to run the following SQL:
select count(*) from ( ( select * from tab1@source minus select * from tab1@target ) union all ( select * from tab1@target minus select * from tab1@source ) )