I’m not sure if there’s a way to see all available in-app purchases through the app store, but most applications have a section titled “Top In-App Purchases” that can give you a sense of the types of additional purchases an app might require. For example, “Stop, Breathe, & Think” is a free app, but the top in-app purchases mainly feature membership and subscription options. It gives you a sense that not all features are available from the base app alone.