Although I've seen many discussions on how to do an interview and develop your brand, I haven't seen many discussions on how to determine if your hiring & interview process is actually working well. I think this question has two parts:

1. How do you know your hiring process is getting the right candidates to apply and go through the interview process?
2. Of the people that you end up interviewing, how can you tell that the ones you choose to hire are better (on average) than those that you rejected?

I suppose the "extreme" cases - when you end up with a superstar or a total dud - are pretty obvious, but what about the rest?