AI writing you can recognize as AI writing is obvious. Newer models are better about this and the line will only get more blurry. Here's a benchmark where good writers make the assessment rather than different LLMs ranking each other: https://surgehq.ai/leaderboards/hemingway-bench

The top models are also the latest:

Gemini 3.1 Pro: still a bit of a gremlin, but will probably stay on top until the other model makers go xkcd 810 and target this benchmark

Gemini 3 Flash: current favorite of writers using it as a helper for its speed and decent prompt following