See how to: Find information about your warranty or AppleCare plan - Apple Support
or
Registered AppleCare plans are listed with your Apple device on My Support.
Learn what to do: If you don’t see your AppleCare plan on My Support - Apple Support
You can enter the iPhone's serial number online to check your service and support coverage
Check Your Service and Support Coverage - Apple Support
Apple Care Purchases and support webpage:
If needed: You may be able to use the app to contact Apple Care about your purchasing bill.
AppleCare Products - Apple
AppleCare Terms and Conditions: Legal - AppleCare+ - Apple
If it is the receipt for purchasing the iPhone that you need to locate:
See if you kept a paper receipt with the iPhone packaging, or filed it away with warranties or guarantees.
You can check your eMails from Apple around the time of purchase for an e-mailed receipt.
If you purchased online check your online account details: Log-in page: https://appleid.apple.com