Turing speculated that the ability to act human would be the best indication of humanlike thought, but a better indication would be the inability to act otherwise.

To clarify: I’m not talking about the popular conception of the Turing test as something like the Voight-Kampff test, meant to catch rogue AIs—but Turing’s original test, meant for AI designers to evaluate their own machines. In particular, I’m assuming the designers know their machine well enough to distinguish between a true inability and a feigned one (or to construct the test in a way that motivates the machine to make a genuine attempt).

And examples of human inabilities might be learning a language that violates the patterns of natural human languages, or engaging in reflexive group behavior the way starlings or fish schools do.