How can I check that my Samsung phone is original and hasn't been modified?
The simplest answer is you can't check if they have changed your hardware or not with comparing build numbers. But the easiest way if something is changed or not will come up in an app called CPU-Z it show all the software and hardware informations that the device has.
I recommend everyone to give it a try before buying a device because it helps too much. I also realized in settings it has validation option you might give it a go. It will probably ask your details to the vendor and email it back to you