With luck, you should never need to check your iPhone’s or iPad’s warranty status. But bad things do happen to good devices. In iOS 12.2, Apple made it easy to see if your device is still under warranty or covered by AppleCare+. Go to Settings > General > About, where you’ll find a new entry that’s either called Limited Warranty (the basic Apple warranty) or AppleCare+ (the extended warranty you can buy).
This entry shows the expiration date, and tapping it provides more details on the Coverage screen. If your iPhone or iPad doesn’t have AppleCare+ but is eligible for it, you can even buy it from this screen. (You won’t see anything if your device is out of warranty and no longer eligible for AppleCare+.)