How do I check the NLS_LANG of the client?

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.

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.key
C:\Oracle10\BIN\oracle.key

C:\>type C:\Oracle10\BIN\oracle.key
SOFTWARE\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.dll

In [37]: OHOMES_INSTALLED
Out[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_HOME
Out[39]: 'C:\\Oracle10\\BIN'

In [40]: f = open(os.path.join(ORACLE_HOME, "oracle.key"))

In [41]: SECTION = f.read()

In [42]: SECTION
Out[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)