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 )
)