I read an interesting paper about an idea of watermarking LLM output text in such a way that makes detection very accurate for a long enough text. This is done by subtly changing the probabilities of the next word to be generated based on the last word that was outputed. Circumventing it by manually changing words post hoc would potentially require almost as much work as writing it from scratch.
The idea seems quite roboust to me and I can envisage a future where companies that provide access to LLMs would also publish a detection tool for their models.