Code often doesn't stand alone, there's going to be a commit history, there's often going to be comments and issues on other projects.

Ask questions about the code:

- why did you write this in this way

- explain how this works

- if you needed to add X into this, how would you do it

- are there other ways to achieve what you did here, what are the pros/cons of this?

- what would you do differently if you were to re-write this?

- are there any bugs or common issues, how do you go about diagnosing and debugging them? Could you write the code differently to be more resiliant to those issues?

Anyone who wrote that code (assuming it was relatively recently) is going to be able to talk about it sensibly, and be able to dig into the code and show how certain things are done.