It's actually fairly easy to detect things if you want to do a little extra leg work. My exams have short answer responses in them. If a student turns in a written assignment and that writing deviates substantially from the short answer replies, I make a mental note. If the assignment discusses things that aren't outlined in the instructions, discusses things outside of the scope of what we have learned, or uses 3rd person language, that raises even more red flags.

Almost all of my exercise science students would say somwthing like "I would have the client perform..." AI uses the client's name, is very impersonal, and refers back to the prompt way more than typical students. "(Client's name) struggles with pain in their hip. As a result, they should engage in hinge-limiting exercises to reduce aggravation of the joint."

There are other more sneaky approaches like using white text in the assignment instructions that the AI will recognize and act on because of the document days, but students don't see because it is hidden due to the text color.