In my Flask-SQLAlchemy web application I have implemented Flask-Login for user authentication and authorization. Now I am building an Admin
You will have to implement this feature yourself as there is not a way to do it natively with flask-login.
I can think of myriad ways to do this, but the quickest is probably just keeping track of the last time a user accessed your site. Then you can set a reasonable timeout where you assume the user is not logged in if they haven't done anything for the last 30 minutes (or however long you feel comfortable with)
For example:
@app.before_request
def update_last_active():
current_user.last_active = datetime.utcnow()
db.session.commit()
This would update User.last_active every time the user sends a request to your server.