[Question] What's the best way to know whether the user is Broadcaster or Viewer?

Extensions
2

As documented:

onAuthorized

returns userID (Opaque user ID.)

and

Broadcasters’ tokens are set with "U" + their Twitch user IDs, to avoid confusing opaque IDs with user IDs when the broadcaster is a viewer.

If that helps.

Or onAuthorized parse the response (don’t validate) and test the role key