Session in “Sql*net message from client”, but is active

I am a little doubtful that this is exactly what you are looking for, but

host echo %nls_lang%;

ENGLISH_UNITED KINGDOM.WE8ISO8859P1

shows the client nls_lang environment variable on the client.

I don't think there will be a SQL query you can run to give the 'current' setting because AFAIK the server is not aware of what translation is done client-side, so any command to show the current setting will have to be native to the client - I used SQL Developer for the above command, but I assume it will work the same in SQL*Plus

--edit

from AskTom:

only the client knows their character set as well -- it is not available "in the database"

and

the character set describes what is stored in database.

the client makes their desired translated to character know [sic] to the database via the NLS_LANG settting.

If you were on 11.1+, you might have some joy with v$session_connect_info, because:

This information is pushed by OCI to the server ats login time.

But I discovered it would still depend on how you are connecting, eg from the JDBC Thin Driver you aren't using OCI and so the information isn't pushed

The Active Session Pool allows a limit to be place on the number of sessions that can be active on your instance. It is a part of the Resource Manager. The number possible is defined according to your current resource plan. The default plan allows an unlimited number of sessions. It is activated if resource management is performed in the instance. To find out if this is so then check the resource_manager_plan parameter:

show parameter resource_manager_plan

If there is a value then it is activated. You may see something like DEFAULT_PLAN as the VALUE. You can then query DBA_RSRC_PLAN_DIRECTIVES for the number of active sessions possible:

select GROUP_OR_SUBPLAN, ACTIVE_SESS_POOL_P1
  from DBA_RSRC_PLAN_DIRECTIVES
 where PLAN = 'DEFAULT_PLAN';

You'll get a few rows back relating to the number of groups/subplans that are part of this plan. NULL will indicate unlimited. I prefer using Enterprise Manager to query and amend this. To find it in EM go to the Server tab then under the Resource Manager heading is Plans.

More information on the Resource Manager can be found here.