The model number, serial number, and IMEI should be displayed on the iPhone box. Phone details are available when you open the Settings app and select General > About. If your details do not match, then your phone may be fake.
Where to verify serial number
The serial number can also be verified at https://checkcoverage.apple.com. Entering an iPhone’s serial number into Apple’s Warranty Status website will provide information about the model, warranty period, support status, and more about the phone.
Check IMEI number by dialing
Check IMEI number at http://www.imeipro.info. Every phone gets a unique IMEI number. Searching that number in the database will give you information about the phone. Dial *#06# on the keypad or check the SIM tray to search the IMEI.
Do physical checking of the phone
Check for the Apple logo on the back of the iPhone. All iPhones have the Apple logo on their rear. A Certified Apple logo should not have a raised or textured feel. If rubbing your finger on the logo feels different than rubbing it on the back of the iPhone, then the phone is bad.