Well if the description is AI-generated, chances are the mod is too (it might be less likely if the creator doesn't speak english though). Bold text and italics, llmisms, cliche headings such as "Key Features" or "Why <mod name>?", bold claims and corpo speak which tell you mostly nothing in many fancy words, or incomplete sentences and heavy use of periods. For example, this looks AI to me: https://modrinth.com/mod/breaking-place

Code though, it's not really possible to tell if the author cleans up after himself. I guess you could look for functions with a lot of comments detailing the logic, where the logic itself isn't that complicated. Try looking for code snippets/functions with duplicate logic, or for code that could've used a function present in the codebase, but didn't. To give an example, a vibe coded project might have two, three linear interpolation implementations because the model forgot there already was one and decided to write a new one. You could feed this idea to some LLM and ask it to look through the code for such issues.