appearance:
The Apple logo-which is at the back of the phone indicates a real iPhone logo.no logo then its fake.if the logo is there when rubbed using a finger a transition from phone cover to the apple logo is felt.
performance:
multitasking-fake iPhone have a hard time supporting to applications at the same time. iPhones run even if multiple applications are opened.
system :
Serial number : dial the code *#06# on your dialer to display the serial number. These should march with the serial number on the phone settings.
iTunes : connect your phone to a pc and try to run it with iTunes. If its an error then it means iTunes do not recognize your iPhone 6 thus undetectable and fake.