I actually don't know how this would become obvious. Do you look at your server screens while they boot? Any attempt to checksum the BIOS from the operating system could be easily bypassed by simply regurgitating the OEM BIOS if anyone reads it.
Do you look at your server screens while they boot?
Long enough to see the POST screen where I put the message:
Award Modular BIOS v4.51PG ==Customised by XXXXXXXX==
is pretty hard to not notice, especially if it suddenly changed back to string in the OEM BIOS. The code I inserted basically computes my string by "encrypting" the original string with some constants derived from the whole BIOS image, so if the BIOS was only partially modified it turns into rubbish, and if it was replaced, then the string reverts to the original one. The point is that the BIOS on that machine is globally unique, has a feature that I can easily identify, and would be difficult if not impossible to forge by an attacker with only the OEM BIOS. I came up with this shortly after the infamous CIH virus emerged.
Ditto for the NSA trying to flash a backdoored Coreboot: they would have to know exactly the customisations I made to my BIOS to replicate them, and if they did, then I'm probably pwned anyway. Backdoors in an offboard option ROM would certainly avoid modifying the BIOS, but I guess a similar mechanism could be used to customise one to easily detect its modification.
