Manufacturers already use x-rays to make sure they're not installing counterfeit chips. Maybe we can look at the verification techniques used for US military procurement or by NASA.
I'd wager that we can have open hardware, and that its correct manufacture can be verified to a satisfactory degree by a combination of auditing and testing without running into limits of the laws of physics or inherent design requirements.
Anyway, it's just different than software. You can't sha sum your hardware to verify its what you expected.
I'm not sure, maybe an x-ray could see inside. Not unlike how binary blobs can be examined, but usually that's not considered open.