login via specified user and run:
select column_name from user_tab_columns where table_name = '<your_table>';


[DBA/ALL]_USERS

Default schemas:
http://abcdba.com/abcdbaserver11gdefaultschema
https://docs.oracle.com/cd/B16351_01/doc/server.102/b14196/users_secure001.htm

select username, account_status, lock_date from DBA_USERS where default_tablespace = 'USERS' and account_status='OPEN';

space occupation:
http://www.sql.ru/forum/941605/kak-otlichit-sozdannyh-polzovateley-ot-oraklovyh
http://remidian.com/2009/08/purging-sysaux-tablespace-purging-awr-reports/