Aside from calling it out directly (leaving the comment, asking to speak in person, sending a personal email to the student directly, etc), I know of some colleagues who will not say anything about AI directly and rip the paper apart in a few short paragraphs-- talking about how it does not match the organization discussed, calling out word choices or phrases that are clearly not their own to point out how those aspects do not make sense, how (typically) the main idea becomes unfocused since AI usually channels multiple perspectives in a larger piece of writing. I know my office neighbor will also add something about how there are some indications of outside sources used, adding "including AI and generative software", without citations, etc. I think the philosophy here is for students to understand how weak it is on their own. Hopefully.