How to check Oracle database characterset (NLS parameters)

I often get questions about Oracle database NLS parameters, especially about characterset, it's very easy to check this parameters with command:
for only characterset

select value from nls_database_parameters where parameter='NLS_CHARACTERSET';

for all nls_parameters :

SELECT * FROM nls_database_parameters;

but many of this parameters are overwritten in session level, hence we can check this parameters in session level with command:

select * from nls_session_parameters;

that's all,
good luck.