Do you need to compare rows in two tables that have the same columns?
I'd use the SQL MINUS operator:
This is just the easiest. A more sophisticated compare might also list rows that have the same primary key but differ in other columns. You'll be much safer if you enumerate the columns instead of using * (see Always use column list).
If the two tables reside on the same database in different schemas, do the compare using an account that was granted the SELECT privilege on both tables. If the two tables reside in different databases, you have several options:
1) Create a database link and run the "minus" queries using the link,
2) Use data pump (expdp/impdp) to bring the two tables into one database, so that you can compare them,
3) Export the data somehow into text/csv files and compare them using different tools (diff, excel or the like).