While you may find a particular attack if you're looking for, in general, it's impossible for even the most thorough audits to check for the whole class of such attacks. You're not going to look into the chips. Well, you can, but that's prohibitively expensive and destructive - even if you could check that this chip was okay, then you still have to throw it out after analysis and plug in a different one.
The only feasible thing to do is thorough audits of all the supply chain for every component in your system, ensuring that your supply chain does not include even a single chip from an "untrustworthy" supplier, and even then it reduces the chances of an attack but does not eliminate it.
> Faggin: Yes, we were concerned about others copying the Z80. So I was trying to figure what we could do that that would be effective, and that’s when I came across an idea that if we use the depletion load the mask that doesn’t leave any trace, then I could create depletion load devices that look like enhancement mode devices. And by doing that we could trick the customer into believing that a certain logic was implemented, when it was not. Then I told Shima, “Shima, this is the idea how to implement traps. Put traps, you know, figure out how to do the worst possible traps that you can imagine,” and then Shima with his mind, that was steel mind, was able to actually figure out a bunch of traps that he could talk about.
> Slater: You want to tell us a little about that Shima?
> Shima: I didn’t count [on] talking about that mostly. I placed six traps for stopping the copy of the layout by the copy maker. And one transistor was added to existing enhancement transistors. And I added a transistor looks like an enhancement transistor. But if transistors are set to be always on state by the ion implantations, it has a drastic effect on very much. I heard from NEC later the copy maker delayed the announcement of Z80 compatible product for about six months.
They likely needed to have the exact official schematic of the motherboard to compare every single detail of the hardware with.