Get channel name after login on website

Barry, I think I am missing something.

Using the Implicit, I get back this:

https://localhost#access_token=0123456789abcdefghijABCDEFGHIJ
    &scope=viewing_activity_read
    &state=c3ab8aa609ea11e793ae92361f002671
    &token_type=bearer

So how can I use this to check if A is my sub?
I obviously have MY client ID but I have not the one of the person who is logging, right?

If the call is this
GET https://api.twitch.tv/kraken/users/<user ID>/subscriptions/<channel ID>
I need to know both user ids