you can use the following command in SQL Developer or SQLPLUS in command prompt to find out the Oracle server version number.
select * from v$version;
in my case it gave me the below mentioned info.
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
PL/SQL Release 11.2.0.1.0 - Production
"CORE 11.2.0.1.0 Production"
TNS for 64-bit Windows: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production