working with sessions

I was wondering if there is a way to tell if a session exists or not. For example, have a system where people login and it starts a session like: sess_cf4c4f6a8cffaf3334df48b6ea1d55e4 If the user logs out or the session is destroyed, then it won't exist anymore. I know you can use the session_id() function after the session is started and in this case it would return: cf4c4f6a8cffaf3334df48b6ea1d55e4 My question is can I search the session folder for: cf4c4f6a8cffaf3334df48b6ea1d55e4 (or any other session id) to see if that session still exists and is active, or if it is gone and doesn't exist anymore? Can this be done? Please let me know if you have any thoughts. Thanks.. Matt