Find My
How will I know if this iPhone has already been registered to Find My by its previous owner?
iPhone SE, iOS 15
Question marked as
See the "Check for Activation Lock before you buy a device from someone else" section of Activation Lock for iPhone, iPad, and iPod touch - Apple Support
For instructions on what the previous owner should do before selling the phone to you, see: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support