You can query the A4DN database to determine if there are any currently logged on users. Here is a query which will tell you the users that are currently logged on and which systems they are logged onto.
SELECT s.FWSYNM as 'System',
u.FWUSUS as 'User Id',
u.FWUSDS as 'User',
l.FWJLONDT as 'Logon Date',
l.FWJLONTM as 'Logon Time',
l.FWJLPCNM as 'Computer'
FROM FWJL l
LEFT JOIN FWUS u ON l.FWJLUSID = u.FWUSID
LEFT JOIN FWSY s on s.FWSYNO = l.FWJLSYNO
WHERE l.FWJLOFDT IS NULL
