There are several ways in which a company can know whether an employee will be a good manager.
First, it should go without saying that an employee should know the company well. In other words, a person must be good at his or her current job and be knowledgeable about the field. This is important, because people will look up to a manager, and one of the ways a person gets respect is through knowledge.
Second, a company should look for how this employee deals with others. In other words, people skills are important. So, a company can look for things like: does his person take interest in others? Does this person have tact under difficult and uncomfortable situation? A recent article in Forbes lists these as some important points to consider.
Third, does this person have organizational skills? Can this person delegate responsibilities when he or she needs to? Does this person work with other in team projects.
All of these are clues as to whether this person will make a good manager in the future.