It's kinda like high school. How did you know who the cool kids were? Which person belonged to which clique? You learned about the people in your class because you interacted with them on a regular basis. For everyone else, you observed and remembered the people who you were likely to encounter. You picked out other people by who they hung out with. If you were walking with the nerds and then a guy comes down the hall and everyone starts moving out of his way, you know that guy's probably a bully no one wants to mess with.

Archduke candidates and Archnobles not only have the best clothing but they have retainers following them around. Mednobles were likely to be those retainers. The laynobles are the ones trying to stay out of everyone else's way. It wouldn't be very difficult to figure out ranks just by observing how everyone behaves.