Before buying a used iPhone, always ask the seller for the serial number. You can plug this number into Apple’s website.
If you still have the box your iPhone came in, you don't need the phone to find the number. check iphone serial number
Using third-party lookup tools (not official Apple), you may also see: Before buying a used iPhone, always ask the
Log in to the Apple My Support portal with your Apple ID. This will list every device associated with your account, showing serial numbers and detailed warranty information for each. Buying a Used iPhone? Do This First Before buying a used iPhone