To distinguish between "is an illusion" and "is not an illusion", you need evidence that isn't observational. The whole point of illusions is that observational evidence is unreliable.

A desert mirage in the distance is an illusion; to the observer, it's indistinguishable from an oasis. You can only tell that it's a mirage by investigating how the appearance was created (e.g. by dragging your thirsty ass through the sand, to the place where the oasis appeared to be).

If one has a reasonable understanding of 2 concepts that make up a larger system. And, such a system has little else in addition to those concepts, one is able to come up with that system by itself. Even though, it has never seen it, or their composition was never explained prior to that logical process.

The illusion happens when, clearly, the alleged reasoning behind how such a system comes to be is based on prior knowledge of the system as a whole. Meaning, its construction/source was within the training data.