Put a mixture of questions in that are impossible to answer but don't tell them, and as usual give them marks for explaining their working. Tell them you've devised the questions so some are "AI resistant" but don't tell them how. They'll never know if the AI has tripped up and hallucinated or given them the right answer. Only those that really understand what's going on will set out some working that shows they were on the right route and then got stuck where they were supposed to.

That will either lead to students becoming frustrated after spending inordinate amounts of time on the unsolvable problem or students giving up too early on the solvable problems.