There's no software method to finding out if your screen is genuine OEM Apple screen that I know of, the only way you can tell for sure is to take the screen off of the phone and follow these steps as shown in the link below. Obviously this would void your warranty since you are servicing your own phone, however your warranty has probably already been void if Apple told you that you have a 3rd party screen, which already voided the warranty.
See this link below for tips on identifying a 3rd party screen vs a legitimate OEM apple screen. The easiest way to tell is if the screen has an apple logo on the flex cable.
http://www.etradesupply.com/blog/5-tips-to-tell-oem-and-copy-lcd-of-iphone-6/
Hope this helped!