One subscription for multiple apps?

[ragcsalo]

Android Developer documentation says: "You can implement your own solution for sharing subscriptions across as many different apps or products as you want. For example, you could sell a subscription that gives a subscriber access to an entire collection of apps, games, or other content for a monthly or annual fee. To implement this solution, you could add your own business logic to your app to determine whether the user has already purchased a given subscription and if so, allow access to your content."

My question: how can I determine if the user has a valid subscription for ANOTHER app (that I own)?