As this is code for an interview the candidate is probably going to bring what they feel is the best representation of their work.
Before delving into the code, ask them about it. Get them to demo it if they can. This helps you get an idea of what they have built and allow you to start picturing how you would have done it.
Next, move to looking at the code. Use the code to start a conversation. If you see something you don't understand, ask them to explain it. If you see something that you would have done differently, ask why they chose to do it that way.
You're not so much reviewing the code as you are reviewing their coding ability and their thought process. Talking about their code gives you both something to talk about, something that the candidate should know fully and that should help calm their nerves a bit.
If the candidate can give reasons why they did something, or can show how their code evolved it will give you a good idea of how strong of a developer they are and if it is someone who will fit into your team.
If the candidate dodges questions, gives vague answers, or can't really answer the 'why's of the code, this is a sign that they might be a weaker candidate.