Comment by blackbear_

Ask them to share their screen, walk you through their code and explain their solution live. Every once in a while ask why they did it in that way and what alternative approaches there could be. Then change the problem a little and ask them to modify their code, again live. Generally, try to go deep and ask follow-up "why" or "how" questions. Those who "don't remember" or only offer vague and shallow answers are likely to have cheated with LLMs (or are just poor candidates).