Apple:
Whether you downloaded Xcode from Apple or received Xcode from another source, such as a USB or Thunderbolt disk, or over a local network, you can easily verify the integrity of your copy of Xcode.
To verify the identity of your copy of Xcode run the following command in Terminal on a system with Gatekeeper enabled: spctl --assess --verbose /Applications/Xcode.app
This is good to know, but shouldn't Apple be able to run on similar check on their side to make sure that an app was compiled with a verified version of Xcode?