If you are just doing this for the Roku platform with Roku billing, for subscriptions, you should be able to get away without using push notifications, even to see if it is active.
Since all subscriptions for the same roku account use the same roku billing system, if you only offer a single subscription option, you can query from within the roku channel app itself if any subscriptions are available for the user to purchase. If yes, then the subscription isn't valid and they have to get one. If no, offer the available option in the app. Along the same lines if you offer more than one option and they are all different subscription plans, then the user will most likely only be able to choose one of them at a time, so you can do a simple count of how many are available to purchase, if all options are available they aren't subscribed, if less than the total options are available then they are valid for one of them. If you need to know if their subscription is valid for outside use on other platforms then your method appears to be correct on how to tell.