Yahoo! Messenger can set your status to invisible to avoid people from irritating you. But still you can see who is online, this is really a good feature, but in-case if your friend is trying to hide from you, he can easily kept his status as Invisible. Then how-to find that he is in invisible mode, there are lots of ways where you can find your friends visibility status. But most of them are paid services, and some of them are need to be installed in your system.
In my opinion that is all crap and wasting money with system resources, you have plenty of tools available for free to do this job, you can find your friends status like Online, Offline or Invisible. Here I am giving a list of working sites which can do this job for free.
Find Invisible Yahoo Users:
These are the websites which provide you this facility to find the invisible status of Yahoo messenger user, please let me know if you have any better online tools which can do the same.