[Calgon]

When a player connects or logs in, update a value setting the player's online status to 1. When the player disconnects, update a value in a table/field setting the player's online status to 0. And in case the server crashes, set all player status values to 0 in OnGameModeInit.

If you don't use MySQL or a similar system, then you're not going to accurately be able to determine if a player is logged in and online, just if they're online using the connection API functions Westie released a few years ago.