I've been at the front of the classroom--using tools like TurnItIn is fine for getting "red flags," but I'd never rely on just tools to give someone a zero.
First, unless you're in a class with a hundred people, the professor would have a general idea as to whether you're putting in effort--are they attentive? Do they ask questions? And an informal talk with the person would likely determine how well they understand the content in the paper. Even for people who can't articulate well, there are questions you can ask that will give you a good feel for whether they wrote it.
I've caught cheaters several times, it's not that hard. Will a few slide through? Yes, but they will regardless of how many stupid AI tools you use. Give the students the benefit of the doubt and put in some effort, lazy profs.
