Apple's docs say
Note: You don't need to create a universal purchase for a watchOS app with an iOS app because the app is already included in the same Xcode project as the iOS app. Learn how to add watchOS app information.
When you make a purchase, no receipt is found on the watch: https://twitter.com/AndresBoedo/status/1237816178057318401?s=20
So other than using WCSession to ask the iPhone if subscribed, how are we supposed to check on the watch?