How to verify that the user is logged into GMail/VK (to offer registration)?

Question

JavaScript

Good day!
The customer wants the user to be prompted to register after viewing several pages,
and if the user is logged in to GMail, then through it, if in Vkontakte, through it, and so on with popular social networks. networks.
I'm interested in exactly how to find out whether the user is already logged in or not and offer him only the entrance through those services in which he is currently logged in (not active ones will be placed under the spoiler).
Preferably without third-party services - only JavaScript.
I thought maybe somehow through a frame, but then I would like to avoid spending on traffic to load the entire VK / GMAIL site into a frame ...
For example, the user is logged into VK, a button pops up and offers to log in through VK.

Answer 1

Read the protocol and implement it yourself =)