Chose a person you want to check .Double click to open the chat window.Now simply invite the person for a voice conference.This is done by clicking on the "conference' button on the top of the chat window.
If the talk button appears and turns green then that person is online.But if you receive the message 'Voice chat could not be started ' then the person is offline.

or

no software needed or anything else
double click on the name of the person to whom u want to chat
then click on any environment doodle
and buzz or ctrl+g if the page load then the other person is online




other ready made softwares are :

spy the invisible
www.xeeber.com




Invisible
Jus go to www.invisible.ir