@chilimatic oh, I don't have quite a lot of experience with that. I usually do stuff on my own and people don't care a whole lot about it, so I constantly have to look up best practices and be critical myself :D
But all in all, when I have to review some task a student works on (I have one or two from time to time), I usually do just what @chilimatic mentioned. I sit down with them, the request on the table in front of us, and then I ask them to explain what they thought while creating the application. They usually explain whatever they did and thought. Often, especially when I don't agree, I ask them about a situation in which things probably won't work out. That way, they start thinking about such a situation and how to manage it themselves.
As a result, I do not criticize them and it is a learning experience!
To be fair: You need to have at least a bit of UX knowledge. If you don't feel too comfy yourself, ask a UX specialist to join the meeting. Plan the meeting with the specialist beforehand!