How do I check the NLS_LANG of the client?

This is what I do when I troubleshoot encoding-issues. (The NLS_LANG value read by sqlplus):

SQL>/* It's a hack. I don't know why it works. But it does!*/SQL>@[%NLS_LANG%]SP2-0310: unable to open file "[NORWEGIAN_NORWAY.WE8MSWIN1252]" 

You will have to extract the NLS_LANG value in current ORACLE_HOME from the registry.All client-side tools (sqlplus, sqlldr, exp, imp, oci, etc...) read this value from registry and determine if any character transcoding should occur.

ORACLE_HOME and registry section:

C:\>dir /s/b oracle.keyC:\Oracle10\BIN\oracle.keyC:\>type C:\Oracle10\BIN\oracle.keySOFTWARE\ORACLE\KEY_OraClient10204_Home

In times like these I turn to IPython to demonstrate an idea:

A couple of lookups and you are there!

In [36]: OHOMES_INSTALLED = !where oci.dllIn [37]: OHOMES_INSTALLEDOut[37]:['C:\\Oracle10\\BIN\\oci.dll','C:\\oraclexe\\app\\oracle\\product\\11.2.0\\server\\bin\\oci.dll']In [38]: ORACLE_HOME = os.path.dirname(OHOMES_INSTALLED[0])In [39]: ORACLE_HOMEOut[39]: 'C:\\Oracle10\\BIN'In [40]: f = open(os.path.join(ORACLE_HOME, "oracle.key"))In [41]: SECTION = f.read()In [42]: SECTIONOut[42]: 'SOFTWARE\\ORACLE\\KEY_OraClient10204_Home\n'In [43]: from _winreg import *In [44]: aReg = ConnectRegistry(None,HKEY_LOCAL_MACHINE)In [46]: aKey = OpenKey(aReg,SECTION.strip())In [47]: val = QueryValueEx(aKey, "NLS_LANG")In [48]: print val(u'NORWEGIAN_NORWAY.WE8MSWIN1252', 1)

According to Jocke's answer (thanks Jocke), I tested the following query:

SELECT DISTINCT client_charset FROM v$session_connect_infoWHERE 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 nAMERICAN_AMERICA.WE8ISO8859P1

Just build a function-wrapper, grant execute to the users who needs it, and there you go.