how to verify iap subscription active?

Hello, New to Unity forums and I've looked over a few posts.. How are folks enforcing having active subscriptions to use an iOS app. I've seen references to Prime31, Easy Plug (which support looks dead) and others..

I don't really need server-server receipt validation, I already create users on the backend for other processes, so I can validate who is accessing the app from a SQL perspective, but I am not sure of what is the best, most sustainable (plugin author support) method for in app subscription verification.

Thanks - Ron