If you want to know what a man's like, take a good look at how he treats his inferiors, not his equals.

This statement is saying that if you want to understand the kind of person someone is, you should look at how they treat people they are in positions of power over, such as their subordinates, employees, or people beneath them in the social hierarchy. This will tell you more about the person than seeing how they treat their equals, since people may act differently when they have power or authority.