how to find the product is not stolen

The instructions on this Apple Support page include how to tell if the previous

owner had released the unit from their 'activation lock' & other account status.

If your question still pertains to the topic where you first posted it; iPhone:

• Find My iPhone Activation Lock - Apple Support

From inside this article, part of the information appears below:

How do I check for Activation Lock before purchasing a used device?

When you buy an iPhone, iPad, iPod touch, or Apple Watch from someone other than Apple or an authorized Apple reseller, it is up to you to ensure that the device is erased and no longer linked to the previous owner’s account.

You can check the current Activation Lock status of a device when you visit icloud.com/activationlock from any Mac or PC.

Or you can follow the steps below to make sure that you can use the device you purchase:

1. Turn the device on 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. After choosing a language, choosing a country, and connecting to a network, the device will begin activation. 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.

Another set of things to check, appear here:

7 Things You Need to Check Before Buying a Used iPhone

Buying a used iPhone may seem ... to consider before you buy a pre-owned iPhone...

that the phone you want to buy hasn't been stolen or reported ...

engadget.com/2016/08/03/7-things-you-need-to-c

When you know the person who actually owns an iPhone, you can suggest they

follow advice by using official steps to get it ready for another owner (you)

•What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support

• If you can't activate your iPhone - Apple Support

Hopefully the information will be useful.

Good luck! 🙂