If you do suspect that an answer is generated by ChatGPT, you can then run it through the detector that @ArturMeinild mentioned. However, the detector is not nerfect (nobodypobody is), and can be (and has been) fooled.
A new user who posts a "professional looking" answer as their first post. That's not to say we haven't had some stellar first answers here, but for most users it takes some time to become really good at taking the time to fully read and understand the question, and to learn how to write a really good answer.
An answer that mentions a particular setting, with a comment from the OP that they couldn't find that setting. ChatGPT is really bad (or good, depending on how you look at it) about just "making up" settings or features that don't exist. It appears to hate to give a "There's no way to do that" answer.
A new user who posts multiple answers in a short span of time. These answers may not even all be here on Ask Ubuntu. Often, the user will create multiple answers across multiple Stack sites in a short span of time, on a wide-range of topics. So if you suspect that an answer is ChatGPT based, look at other answers from that user, both here on AU and other SE sites.
I came across an answer here on AU just last night that "sounded" ChatGPT'ish, but actually came up as "98% real" on the detector that @ArturMeinild mentioned. But on further examination, I found 22 answers that the user had posted in 7 hours across more than a dozen Stack sites. Fortunately (for purposes of "proof" at least), all of the other answers did test at a nearly 100% GPT rate or highly so.
An answer that doesn't seem to actually answer the question that was asked, or skips key details from it. Often, to get a "focused" answer, someone will just copy the question title (or a subset of the question) into ChatGPT. The answer that is posted will seem "off", if you read it in context of the question that was really asked.