Reputation, recommendations, and references work well. You can contact the previous company or previous co-workers if you don't know much about a candidate. I

Reputation, recommendations, and references work well. You can contact the previous company or previous co-workers if you don't know much about a candidate. I know some companies are very secrets but in practice it's often possible to get a "yes you can hire this person because..." or a "you don't want to hire this person because..." where I live.