Nick's characterizations of people at the party demonstrate his judgments. First, Nick notices that people hadn't really been invited, they just knew to go there. He points out that he himself had been invited putting division between himself and the others there.
He describes people to the audience by their actions or appearances. He noted the Englishmen in their fine suits dotted about the area, he called another man Mr. Mumble because he could barely distinguish the name a person introduced to him was called. He described a drunk man he met with deep eyes as Owl Eyes.
As the party continued he gave the group characterized descriptions by their behaviors. He noticed the ocean of people as they moved about in waves. He also noticed that the appearance of the people was a distinguished group like that of an East Egg crowd. People whispered or shrank away from mentions of Gatsby and Nick could read the apparent mystery about the man.