Errors can give away that a human typed something, but knowing proper grammar, spelling, and syntax of English is totally neutral—if not to be somewhat expected from a native speaker/typer with a lifetime to learn the language they speak (especially if we consider how many Anglophones are monolingual + educated + have access to technology like spell check meaning there is little excuse for not having English mastery).
In my education, I got a public apology from a teacher letting the class know they tried to dig up proof of plagiarism in my persuasive papers, but for the first time proved themself incorrect on a plagiarism hunch. Humans are capable of writing well.
20 years with Fx*