Hi, Im curious on how apple can actually tell if your screen has been replaced if there is some hidden apple serial number or something like that (for iphone)
It doesn't say as much, but if phone is damaged in process, that's not covered: "Important: Do not open the Apple Product. Opening the Apple Product may cause damage that is not covered by this Warranty. Only Apple or an AASP should perform service on this Apple Product." http://www.apple.com/legal/warranty/products/ios-warranty-document-us.html
If I'm not wrong, there is this QR code that they scan on the back of the display when they open up your phone to check for any unauthorized modification/repair.
Chances are your warranty is voided when it's repaired by a third party