How do I check if this iPhone is fake or genuine?

Also see: https://discussions.apple.com/docs/DOC-250001038

Hello~ Click on below and put in the information required….

Check Your Service and Support Coverage - Apple Support

Posting the serial here or anywhere else isn’t a good idea.

~Katana-San~

There are many more things you should be concerned about when buying a phone from a third party, other than it being fake. Many of these things will result in the device being totally unusable and you will have just purchased a brick. With any third party sale, make sure you have the option to return the device. If you don't then you need to purchase from a different location unless you are willing to take that risk.

The All Too Common SAD Reality of Buying … - Apple Community