How to check, where the iPhone is real or not?

If you have the iPhone X you want to buy right now, you can check if it's already used by someone else from this link:

https://checkcoverage.apple.com/gb/en

The website will ask you to enter the device's serial number, if you don't know where is it, follow these steps:

• Open Settings
• Tap General -> About
• You will see your Serial Number, press and hold the number until the screen shows a Copy button.

After you entered your serial number to the website, click Continue.

The website will show you:

• Purchase Date
• Telephone Technical Support status
• Repairs and Service Coverage

If the Purchase Date of the phone is valid, it means that the iPhone X is already used.

I hope this will help you.

The All Too Common SAD Reality of Buying a used iPad/iPhone - Apple Community