Flask-Login user status monitoring
FYI, Counting Online Users with Redis | Flask (A Python Microframework) - http://flask.pocoo.org/snippets/71/.
You could get away with checking if users last activity time is bigger(older) than session life time.
if that's the case, you will go on an update their is_online status.the way i have handled the problem in my application was, since i had a last_activity field in db for each user, to log when they have done what they have done, i could check that value vs session life time.
One very crude method is to create a separate stack that pushes and pops when a user logs in. Assuming that session id and userid on your db is not tied together (i.e., you have separate session id and user id), you can maintain a ledger of sorts and push and pop as sessions are created and destroyed.
You will have to put special emphasis on users running multiple sessions on multiple devices...which is why i put a caveat saying this is a rather crude method.