I'm not a native speaker and my level of AI recognition is higher than 99.999% of native speakers - and I'd be happy to be tested on it for proof.

The biggest factor is simply how long you've been using LLMs to generate text, how often, how much. It's like how an experienced UI designer can instantly tell that something is off by a single pixel off upon first seeing a UI, whereas if you gave me $200 to find it within 10 minutes I might well fail.