Knowing a client PC working with internet !

Hi anyone,

I'm working on a client/server app by delphi7,
and i want to know how Sysop pc know a client pc is working with internet, my mean is when a user on a client pc want to open a website or chatting with someone or downloading something,
that mean is user on client pc is working with the internet, now
in this status, client pc must send a report to server app
(Sysop PC) that it's mean user is working with internet.

How can i know this status?
Any tips is interested