If your box says iPhone 7 Plus it is a fake as the real one has just iPhone written on it.
If there are not any apple stickers you know that you are dealing with a fake. This difference is applicable to all other iPhones.
If you buy the iPhone new and there is not any setup to use the phone, it is fake.
If the home button clicks it is fake. The real iPhone has a sensor instead now.
If you have any questions feel free to contact me at firstname.lastname@example.org