As others wrote, examples, examples, examples. And ask for code to be thoroughly commented. And data! Each variable, each constant, sizes, limits, caveats, trap

As others wrote, examples, examples, examples. And ask for code to be thoroughly commented. And data! Each variable, each constant, sizes, limits, caveats, traps etc. must tell you what they do and why: you aren't just getting the working code, you are getting also the bugs, minus the subconscious inhibitions that prevented the old developers from making them appear. Learning what the code does will be hard by itself, but debugging without proper documentation can become a nightmare.