check for inactive sessions

Ok, I've developed a system that allows N number of users to log-in to my site under the same username/password combo. When a user logs-in, a flag is set marking that they have logged-in, their session id is stored and the current # of user's logged-in is iterated by one. If a user account has a maximum of 5 possible simultaneous user log-ins, the user stack will be full unless those users logout (which will reduce the stack by one), or their session expires. The problem being that I do not know how to check if a session is active, other than checking the current user session of course (not all session's currently associated with an account). I am storing all sessions associated with an account. Any ideas? Thanks in advance Christian