Mark Lloyd wrote:

I remember a lot of ads for "386 cache system", although I don't know
anything about "fake cache chips". What would one do?

Nothing. It was just a plastic DIP package, with some legs and usually
"write-back" printed on it to look good, but no silicon inside, I'm
surprised google can't summon up the X-rays that existed at the time
proving it.

Eventually cache testing programs came along, much like those that can
spot counterfeit large USB flash drives, that are really just lying
small USB flash drives.