Verbatim copies are often not that hard to detect in a large number of cases; there is a tendency to waffle excessively in a way that few people do. This is of course not a fool-proof method, but it's "reliable" in the sense of "you can get it right more often than not".

Edited LLM text is much harder: using a LLM to generate some text and then edit it in shape (often by removing extraneous paragraphs, maybe rewriting a few things slightly). Those are basically impossible to detect reliably.