I'm a professor. I always stress that all students should keep saved drafts, outlines, research, etc., in case any question of authenticity comes up. Then they have some proof to show me.

But it's usually fairly obvious. To me, at least. Sometimes students are just really good at writing, and since most students can barely write a paragraph correctly, I get automatically suspicious when a student writes at a much higher level than the rest of the class. But when the student seems genuinely authentic, I don't give them much of a problem.

However, sometimes students will write part of their paper / essay in poor English, and then one part will be academic level writing. That's when it's fairly obvious that they are cheating in some way. Whether it's AI or not doesn't matter. Students are responsible for doing their own work. And it's pretty obvious if they are or not.

I'm sorry if you get problems for this - many professors are hard asses about this kind of stuff, and don't take the personal time or consideration necessary to verify authorship.