Pre-own iphones

1. Ask for the original Invoice / Bill from the owner.
2. Take the Serial N umber from iPhone, Check here --> Check Your Service and Support Coverage - Apple Support
3. Check the Model Number

The first letter in the model number represents the source of the device. Currently, there are four known possible choices:

• • "F": refurbished unit
  • "M": retail unit
  • "N": replacement unit
  • "P": personalized (engraved) unit

There are no other way of knowing it's originality.

First thing before checking originality

Check for Activation Lock before you buy a device from someone else

Before you buy an iPhone, iPad, iPod touch, or Apple Watch from someone other than Apple or an authorized Apple reseller, make sure that the device is erased and no longer linked to the previous owner’s account. Ask the seller whether or not they've turned off Activation Lock, then follow these steps:

1. Turn on the device and slide to unlock.
2. If the passcode lock screen or the Home screen appears, the device hasn't been erased. Ask the seller to completely erase the device by going to Settings > General > Reset > Erase All Content and Settings. Don't take ownership of any used iPhone, iPad, or iPod touch until it's been erased.
3. Begin the device setup process.
4. If you're asked for the previous owner’s Apple ID and password, the device is still linked to their account. Hand the device back to the seller and ask them to enter their password. If the previous owner isn't present, they can remove the device from their account by signing in to icloud.com/find. Don't take ownership of any used iPhone, iPad, or iPod touch until it has been removed from the previous owner’s account.

You will know that a device is ready for you to use when you're asked to "Set up your iPhone," “Set up your iPad," or “Set up your iPod” the first time you turn it on.

If you buy a device that's still linked to a previous owner's account, contact them as soon as possible and ask them to erase the device and remove it from their account. Learn how to remove a device from a previous owner's account.

https://support.apple.com/en-in/HT201365