When you start SQLPlus you should see the Oracle version information. For example:

$ sqlplus fkim/fkim@example

Connected to:
Oracle9i Enterprise Edition Release 9.2.0.1.0 - Production

Also you can execute SQL to retrieve the version information.

SQL> select * from v$version where banner like 'Oracle%';

BANNER
----------------------------------------------------------------
Oracle9i Enterprise Edition Release 9.2.0.1.0 - Production