I played around for a while asking Chat GPT whether this or that content was generator by it, and there are some certain circumstances where it declares having come up with the text, or provides the "original text" (or a variation) if the data you feed it differs somewhat. The post you referenced was verbatim, so it was an easy match for it to find, but the more you twist it the sooner it declares it not being his. Obviously this is only an indicator though, not a viable declaration of guilt that can be accepted as proof.

If you ask it the same question multiple times it provides different answers, but the essence, and even some fragments, are the same from one iteration to another. A unique pattern will be hard to find, though it does tend to be rather prosaic, often tries to play the neutral role, and for now will generally lack the human touch to its answers, being rather consistent in its posts, as opposed to what one would really be creating by writing posts with a wide range of tones, content length and style.