If your database is set up "out of the box", without special audit set up, you can not identify easly what has been changed.

Normaly, Audit or Fine Grained Audit are used for this purpose.
By default, database records only SYSDBA connections.

BUT, you may try to use Log Miner technology, where special process "dig" the archive logs to find the changes done to the tables.

However, if changes were non-logging or your database is not in archive mode - logs mining will be impossible.