I'm working on Windows OS, I know that this setting is stored in the registry. The problem is that the registry path changes from version to version, browsing though that bunch of registry keys is definitly not a good idea.
I can get the NLS_LANG of the server with SELECT USERENV ('language') FROM DUAL.
I'd like to compare that with the client setting and show a warning when they don't match, just like Pl/Sql Developer does.
According to Jocke's answer (thanks Jocke), I tested the following query:
SELECT DISTINCT client_charset FROM v$session_connect_info
WHERE sid = sys_context('USERENV','SID');
It perfectly does the job, but I'm unsure if any user will have the necessary rights.
I am not sure if this works every time but for me in sql*plus:
variable n varchar2(200)
execute sys.dbms_system.get_env('NLS_LANG', :n )
print n
AMERICAN_AMERICA.WE8ISO8859P1
Just build a function-wrapper, grant execute to the users who needs it, and there you go.