Yahoo messenger is famous chat client with lots of features. While chatting with others you can set your status as online or invisible. When you set your status as invisible your status will be shown as offline to your friends. Your status will pretend that you are offline even if you are online.
There is no official way to detect invisible users on Yahoo messenger. But there are many third-party tools to for this purpose.
CheckInvisible is a simple online tool to find invisible users in yahoo messenger. Just type that Yahoo Id in the box and click Check. It will show you whether the user is online, offline or invisible. Isn’t it cool ?