How do I know which iPhone is original and fake?

You will not be able to tell by looking at the device or a picture of it. That is why it is only recommended to by from Apple or Authorized Apple seller. In addition to being a fake device, there are many more problems associated with buying a device from a third party.

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

Purchase an iPhone from Apple or an Apple authorized service center and you will be safe knowing you have an official Apple phone.