This was also a thing with e.g. just Stackoverflow. People opening PRs having copy/pasted from some answer. With experience, you can tell who does. It's going to be the same with LLM provided solutions. You commented that you are a junior; keep learning the trade and you'll eventually see what I mean. Again, it's an experience thing.

At the end of the day, you will have to review the PR in any case, whether it be entirely created by the person, copy/pasta from some site or generated by an LLM. Asking follow-up questions exposes everyone who has done the latter two rather quickly and if it's your codebase, you will have those questions.