You would need Receipt validation.
If your app offers auto-renewable subscriptions, you can receive server notifications from the App Store about key events by setting up an optional URL that links to your server.
Before sending a notification to your server, the App Store will try to establish a secure network connection with your server by using App Transport Security (ATS) protocols. If a secure connection cannot be established, notifications will not be sent to your server. See Requirements for Connecting Using ATS to learn more about security requirements.
After a secure HTTPS connection has been established, the App Store will deliver JSON objects through an HTTP post to your server for key subscription events. See the In-App Purchase Programming Guide to learn more about the contents of this HTTP post.
Use server notifications together with Receipt Validation to validate a customer's current subscription status and provide them access to content or services from within your app.
For more info : Link