How to check if it is the genuine phone before buying?

First of all check the serial number: Find the serial number or IMEI on your iPhone, iPad, or iPod touch - Apple Support

You will find the serial number and the IMEI. Now you can check the serial number by visiting apple website. If you get the message that it is not valid, then it means your iPhone is not original.

Check Your Service and Support Coverage - Apple Support

If you're buying a used phone and you don't trust the seller, don't buy it.

It's used.

Or at least, it is NOT new.

Buy a phone from an authorized retailer in your country.