By seeing how can take the most punches to the snout. After all, punching someone in the snout is the best way to assert dominance.
Edit: To all of you who think that a Spartan could take a Space Marine... You are fools! A Space Marine is LITERALLY bred to do nothing but fight their entire life. Taking on a 10 ft tall alien with claws bigger then you are? Daily business for a Space Marine. Not to mention they have swords that cut through armor like butter, and can spit acid. That's right. They can spit acid. What are you gunna do about that? That's right, nothing.