Profile → Settings → at the top, you will find your subscription status (here is an example of a lifetime subscription).
Another way for iOS (if you have an annual subscription): Device Settings > click on your account > iTunes & App Store > click on your Apple ID at the top > View Apple ID → Subscriptions > Select App in the Air from the list, and there you can see when your subscription runs out.
You can check this as follows: Select an upcoming flight, click on its "Flight Alerts" widget, then click "Configure" (or "X More"). If flight alerts are available, and you don't see a screen asking you to make a purchase, then you're at a premium.
Another way on Android: Go to our app → Open your profile and click on the icon in the upper right corner → Settings → Scroll down a bit and there you will find "Manage Subscription".