IME: the only valuable signal comes from direct personal referals. If you have someone who you think is good, and they recommend someone who they say is good from a previous engagement, odds are it will work out. There's a transitive, holistic measure in play; they're not going to destroy their reputation by recommending a weak player or even a strong jerk. The problem here is scale (you quickly milk direct networks dry) but nothing else seems to work well.