Pictures below show the places where invisible mode can be activated.
However if you are the one who wants to talk to someone 'hidden' (or the avid admirer spying on their crush's online habits) then the functionality becomes a liability. It becomes a necessity during these instances to find a way to confirm if that person is indeed offline or in invisible mode. Luckily, there's a website that can help check the status of your contacts.
The website's name is statusdetect. Check out the video demonstration of how to use it.