To determine your national character set in Oracle v10.2 (used by NCHAR, NVARCHAR2 or NCLOB), you can use:
select * from v$nls_parameters where parameter='NLS_CHARACTERSET';
Richard also shared another way to get related info via:
select * from nls_database_parameters where parameter='NLS_NCHAR_CHARACTERSET';
which returned "AMERICAN_AMERICA.US7ASCII" and "AMERICAN_AMERICA.WE8ISO8859P1" respectively for the databases mentioned, so apparently that reports on the normal character set only. There is a lot of great info about this here in Oracle in World. If you need help choosing your character set, check this out. Also one of the answers comes from here.
select userenv('LANGUAGE') from dual;