“Passes the Turing test” probably is the best definition we have, but there, there are strategies that make that fairly easy, such as pretending to be paranoid or at an extreme end of the autistic spectrum.
https://en.wikipedia.org/wiki/PARRY:
“A group of experienced psychiatrists analysed a combination of real patients and computers running PARRY through teleprinters. Another group of 33 psychiatrists were shown transcripts of the conversations. The two groups were then asked to identify which of the "patients" were human and which were computer programs. The psychiatrists were able to make the correct identification only 48 percent of the time — a figure consistent with random guessing.”
PARRY was written half a century ago.
esp. of statistic math