I don’t use tmi.js for API stuff myself, but it it passes the response as well as the body back in the callback, then you can look at res.statusCode.which will give you the status code of the request.
If the OAuth doesn’t have the correct scope for the channel and endpoint you’re requesting you’ll get a 403 back, indicating The current authorization is not valid for this context. If it’s a 422 then for the subscription endpoint that’ll mean the channel doesn’t have a subscription program (and so you can’t check for the subscribers list or a specific users subscription because there is no subscription program to check).
If everything worked you would have gotten a 200 status code and then gone on to parse the body.