Give people the chance to take on leadership roles and temporary supervisory opportunities to see how they react. Observe where they do well and where they struggle. When I worked at PricewaterhouseCoopers, it was common to take on the position of acting manager of a single engagement while serving as an associate on all of your other engagements. This practice provided the chance to receive mentoring and guidance as well as to understand the challenges from the inside out, before ever stepping up to the promotion.