SQL & PL/SQL

For those of you who have to select, or participate in selecting, PL/SQL developers, how do you know a really good one when you see him. I am not so much interested in ancillary skills like process modeling, data design, and so forth. I am only interested, at the moment, in PL/SQL skills. What traits tell you this one is a real winner and that one is lacking? Do you use practical questions, a quiz, request to look at sample code?

And, if you can manage to tell the wheat from the chaff, can you also tell who has real promise - those that could become great with a bit of time and mentoring - from those who will likely never be very good?