You should do this check if you are sending an interaction from your Frontend to your EBS anyway. In case the sub started (or lapsed) before the onAuthorised JWT is refreshed updating the client side flags (never trust user submitted data!).
But for the client side stuff, it’s best tested on the actual Twitch website (the developer rig is completely optional, it just saves a need to faff with Self Signed SSL certs, so all my Extension local development is behind a real SSL cert again)