I followed the guide from Google developer document: https://developer.android.com/google/play/billing/integrate?hl=vi

Finished step process Purchase Flow, and handle Purchase result and handle Acknowledge Purchase Response.

And currently I don’t have any backend for doing next step of the guide : https://developer.android.com/google/play/billing/backend?hl=vi

When running the app again, how can I determine user had purchased subsciption already ? And how to determine user subscription when they changed device ?