How to detect and verify a renewal for an auto-renewable subscription?
My experience. Let's assume, we always send initial receipt to Apple's server.
In any case, you'll get JSON with at least two fields: status (no comments) and receipt (information about receipt that you've send).
Additionally to that:
1) If the subscription is still active, you'll additionally get latest_receipt (base64-encoded string) and latest_receipt_info (information about that receipt).
2) If the subscription is already expired, you'll additionally get latest_expired_receipt_info (information about last renewing receipt). Yes, you get only information about it, no base64-encoded string.
And yes, AFAIK, that's not documented anywhere. Hope that helps.