我如何知道用户是否在iOS上通过应用内购买取消了自动更新订阅?

I have been testing out Apple's In-App Purchases (sandbox mode) which is great for testing what happens when a user subscribes, but how do I tell if the user is still paying? How do I know if the subscription has been canceled? Apple doesn't appear to let me test that out?I have been testing out Apple's In-App Purchase