For the Check User Subscriptions endpoint, you would need Ibai to go through your OAuth flow and grant you permission to view who they are subscribed to.
Alternatively there’s there Get Broadcast Subscriptions endpoint which would need the broadcaster to go through your OAuth flow, which in the case of your example would be auronplay.
You can’t check who a user is subscribed to, or who subscribes to a broadcaster, without the respective permission being granted from them.