It's a bit like trying to detect whether you are running in a virtual machine or not. You can't trust a chip to run a piece of code to introspect on itself; instead a more reliable characterization of a chip may come from examining how it fails when you throw it stuff that is out-of-spec.

For example, glitching the clock may lead to a repeatable behavior that could only be produced if no additional circuitry has been added to the critical path that leads to the glitch.