Detect Invisible Yahoo Messenger User

In detecting invisible yahoo messenger (YM) user,
You can choose these links :

http://www.invisible.ir/

http://www.xeeber.com/

http://www.yahootracker.com/

I choose http://www.invisible.ir/ because the server is not always busy.

You can also use softwares like Buddyspy.

Thanks to: Syamim, El Magnifico