There's a very common "style" to ChatGPT responses1. If you start to use ChatGPT yourself, you'll quickly become familiar with it. And ChatGPT can be a useful tool, if you keep in mind its limitations, so I do recommend that you try it, especially while it is free.
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 (nobody is), and can be (and has been) fooled.
Other indicators that increase the odds of an answer being from ChatGPT are:
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 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.
None of these absolutely means that the answer is ChatGPT-based (or AI-based), but each warning flag increases the odds. And it's highly unlikely that a user is going to post just one ChatGPT answer. Over the long term, I'm hopeful that we'll be able to spot the patterns above (and more) across multiple answers.
1 There are a few frequent users here who actually have that same style (and have had, since well before ChatGPT). @karel, @rinzwind, and @NotTheDr01ds (err, yes, that's me) all often have a similar style to ChatGPT. However, we always (in my experience) pass the "real" test on the Detector.
At this point, I commonly scan through the list of new answers in chronological order. I can often spot potential ChatGPT answers just from the first few lines in the preview. However, most of the time when @karel or @rinzwind answer, it often "triggers" my sense of a ChatGPT answer. But it's clearly not, of course.
I've even started to read some of my answers when it took a split-second for me to realize that I'd written it, even though it initially sounded ChatGPT'ish.
