Well, its not too difficult to find if someone in your yahoo messegner’s  friend list is really offline or invisible. Just go to http://www.ydetector.com/ ,  mention yahoo user id of the person(if mail id of the person is tech2know@yahoo.com then just specify tech2know), press ‘Enter’ and bingo!! The secret will be revealed :) . Below is the screen shot of the same.

Yahoo Dectector                                                                                                                      

Stay tuned for more.