On a similar note, in my experience (and this is mainly in C++), reading someone else's code means dealing with hundreds of files and tens of thousands of lines

On a similar note, in my experience (and this is mainly in C++), reading someone else's code means dealing with hundreds of files and tens of thousands of lines. Getting the system in your head is important, but this is difficult to test in an interview -- because it might take days, not minutes. My favorite interview question is asking about projects done in spare time. This yields a great deal of information about how inherently interested the person is in hacking.