How to find out which users are logged on to an Apps instance

FND_USER table stores the details of all the end users. If we give this query:

select user_name,to_char(last_logon_date,'DD-MON-YYYY HH24:MI:SS')
from apps.fnd_user
where to_char(last_logon_date,'DD-MON-YYYY')=to_char(sysdate,'DD-MON-YYYY');

USER_NAME TO_CHAR(LAST_LOGON_DATE,'D
------------------------------------
GUEST 05-FEB-2008 16:01:47
SYSADMIN 05-FEB-2008 16:02:06
USER1 05-FEB-2008 07:31:19
USER2 05-FEB-2008 04:58:12
USER3 05-FEB-2008 09:46:00
USER4 05-FEB-2008 05:00:38
USER5 05-FEB-2008 08:45:07
USER6 05-FEB-2008 09:09:17

The above query can give you a good idea who is logged on.

For a more accurate result, refer to metalink note 269799.1 which says:

You can run the Active Users Data Collection Test diagnostic script to get information about all active users currently logged into Oracle Applications.This diagnostic test will list users logged into Self Service Web Application, users logged into forms, and users running concurrent programs. Please note that to collect the audit information about the users who have logged into Forms, the "Sign-On:Audit Level" profile option should be set to Form. You can access this Diagnostic test via Metalink Note# 233871.1.