In my experience, this is definitely the best way to evaluate how competent someone is in an area. If they can easily hold a casual conversation and express / defend some sensible opinions about an area, it's likely they know what they're talking about. Obviously, you should ask technical questions in the interview, but it's the little things in the conversation that say a lot. Unfortunately, this approach doesn't scale to the size of the AP Exams.

Dijkstra exclusively gave oral final exams on a board. Allegedly, he stopped one of his former students after the first question and told him something to the effect of "you clearly know what you are doing by how you answered that, so you'll get an A, but your handwriting is atrocious." He spent the rest of the exam time making the student practice penmanship.