Being able to walk through someone else's code is a critical skill that a lot of people take for granted. I am not a big fan of the whiteboard during an interview and like to create environments that mirror what they would do on the job. Walking through a tutorials code is one of them. The other thing you can do is have them write a fizz buzz program, at home after they come in for an interview. But if you do this, you should reserve it for candidates that have no code to show but that you are strongly considering picking up. I would have no problem writing a fizz buzz for a company that I know I am in the last round of hiring with so long as I can do it in my environment.