At any forum , you can see if a specific member is online or offline, or generally , how can I detect online visitors accurately who are visiting my website at the moment ...

Thanks in advance

That is typically done by logging the time when a given user (or a guest, if you want that) was last seen.
Once a user hasn't been seen for some predetermined amount of time, he is considered off line.

This is really the only reliable method, given the stateless nature of the web.

Have a look at http://www.hscripts.com/scripts/php/usersOnline.php and play about with that code.
Regards,
Sam Rudge

This was actually the script I was going to suggest. You could implement it with your current system and possibly identify the IP addresses by the IP addresses your users logged in with. Just a thought.