get character set in Oracle Database
SQL> SELECT value$ FROM sys.props$ WHERE name = 'NLS_CHARACTERSET' ;

or

SQL> SELECT * FROM NLS_DATABASE_PARAMETERS;

Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.

Total System Global Area    3.2E+09 bytes
Fixed Size                  2257520 bytes
Variable Size               2.1E+09 bytes
Database Buffers            1.1E+09 bytes
Redo Buffers                1.7E+07 bytes
Database mounted.

SQL> alter system enable restricted session;

System altered.

SQL> ALTER SYSTEM SET AQ_TM_PROCESSES=0;

System altered.

SQL> alter database open;

Database altered.

SQL> alter database character set WE8ISO8859P15;

Database altered.

SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area    3.2E+09 bytes
Fixed Size                  2257520 bytes
Variable Size               2.1E+09 bytes
Database Buffers            1.1E+09 bytes
Redo Buffers                1.7E+07 bytes
Database mounted.
Database opened.

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

VALUE$
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
WE8ISO8859P15

1 row selected.

SQL>