How to count logged in users?

Well. I once tried to find in the sql-select-result from CI_sessions when a user was logged out... but didn't grasp (it). Now I wonder must I use CI session-helper or is it OK to use php's $_SESSION, in order for all needed info to be stored in the CI_sessions table... I also wonder if the "state" (logged_in or not) of a user can be globally recognised, or if it's read with help of users cookies.