I agree. The only way you can clearly establish a coders credentials in a short amount of time is to look at their portfolio. See what work they've done in the past, if it's up to snuff, and have them explain what they did and why. (People might not
for a reason -- i.e. they inherited a code base like yours. Best if they explain themselves and you don't jump to conclusions -- you might pass over a great coder!).