Determine database character set

You can  run the following SQL to determine whether your database character set is UTF-8. 

SELECT value$ FROM sys.props$ WHERE name = 'NLS_CHARACTERSET' ;

It should return the value AL32UTF8. 

Another option is to run the following SQL :

SELECT * FROM NLS_DATABASE_PARAMETERS;

The value against NLS_CHARACTERSET should be UTF8.It will indicate also the national character set.