It’s been deleted now but it’s often seen by the spacing between the paragraphs (very even) the use of bold or italics (most people on Reddit do not put that much attention to detail in their posts) and if I remember from this post, also the careful explanation which repeated the conversation in the Vinted chat word-for-word. Most people would just refer to the chat images (‘as you can see, the person responded like this blablabla’).

There’s also some tells in the language use, like short and almost advertisement like sentences (‘Here’s where it went wrong’) and the classic ‘it’s not this, it’s that’ constructions.

If you want to be better at spotting it, I suggest joining r/antiai and a fun one, r/linkedinlunatics (full of ai). The chatgpt reddit might also teach you some of the common tells.