To get an overview on the currently loged in users and their session properties the "Session Management" link in the admin area can be used.
Some general information about all active sessions are displayed, e.g. how many users currently are loged in, how many agent and customer users are using the system, e.g. With the "kill all sessions" button all users sessions can be deactivated. Also detailed information for every session are available and every session can be removed separately.