With such a huge market share, the "fake iPhone" market has become very lucrative for unscrupulous phone manufacturers. Some of the earlier iPhone knock-offs are so poorly done that it is easy to differentiate a fake iPhone from a genuine one. However, over time, some fake iPhones cloned so well that they are physically very similar to a genuine iPhone. From the size of the device, the buttons placement, the Apple logo, and other physical indicators of an original iPhone, some fake iPhones are really good imitations.

I got my Iphone 7 plus 30,000 less from the price at Slot Limited, so I was quick to ascertain if i got original apple product.
As I was told, I got the serial number by dialing "*#06#" on my dialer which display my iPhone's serial number.

I accessed Official Apple Support site as disclosed https://checkcoverage.apple.com/ where I entered my iPhone serial number to see if it is a real iPhone.

after entering your serial number on the website, If the site says that the code is invalid, then your iPhone is a clone.

I got below statement when I checked the status o

Eligible for an AppleCare product
Purchase an AppleCare product to get additional repair coverage and technical support.
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Valid Purchase Date
A validated purchase date lets Apple quickly find your product and provide the help you need.
Telephone Technical Support: Active
Your product is eligible for complimentary telephone technical support.
Estimated Expiration Date: October 30, 2017
More about complimentary support opens in a new window
Contact Apple Support

Repairs and Service Coverage: Active
Your product is covered for eligible hardware repairs and service under Apple’s Limited Warranty.
Estimated Expiration Date: July 31, 2018
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Set up a repair

you can Determine Fake vs. Real Iphone through this method

You can try this guide: https://www.unlockunit.com/blog/how-to-spot-a-fake-iphone-7
It's for the iphone 7, but the instructions are pretty much similar.