Often it was possible to tell these apart on repeat interactions.

> a post from a non-native speaker

In my experience as an American, US-born and -educated English speakers have much worse grammar than non-native speakers. If nothing else, the non-native speakers are conscious of the need for editing.

I can always tell the difference between a non-native English speaking writer and somebody who's just stupid - the sort of grammatical mistakes stupid people make are very, very different than the ones that people make when speaking a second language.

Of course, sometimes the non-native English was so bad it wasn't worth wading through it, so that's still sort of a good signal.