What quotes from The Great Gatsby by F. Scott Fitzgerald show that Nick is a judgmental person?
In Chapter I, Nick says that he "snobbishly repeat[s]" something that his father used to say: "a sense of fundamental decencies is parcelled out unequally at birth." This shows that Nick believes that some people are lacking in such fundamental decencies -- and he's referring to people like Tom and Daisy -- and this is judgmental on his part (even if it is true).
Also, in Chapter 1, when Tom and Daisy show some interest in Nick's rumored engagement from back home in the Midwest, he says,
"Their interest rather touched me and made them less remotely rich -- nevertheless, I was confused and a little disgusted as I drove away."
Tom had gone on about his concern that the "white race" was being "utterly submerged" by people of color, ironic for a man who has all the power and privilege the world can afford, and Jordan had revealed that he was unfaithful to Daisy. Daisy had hardly been better behaved. Nick's being disgusted by them seems like the natural response of someone who lacks the sense of entitlement that they possess, but it is judgment nonetheless.
In Chapter II, after Tom and Nick have picked up Myrtle Wilson from her husband's garage and brought her to New York City, she and Tom go into the bedroom of their apartment and have sex while Nick waits in the living room. When they come out and other guests begin to arrive, Nick comments about her change in clothes and how,
"With the influence of the dress her personality had also undergone a change. The intense vitality that had been so remarkable in the garage was converted into impressive hauteur. Her laughter, her gestures, her assertions, became more violently affected moment by moment, and as she expanded the room grew smaller around her, until she seemed to be revolving on a noisy, creaking pivot through the smoky air."
Nick judges Myrtle for her affectation, the way she becomes a total snob when she's with Tom and dressed to impress.
In Chapter IV, as Gatsby tells Nick his "history," Nick has a hard time keeping it together. He says, "With an effort I managed to restrain my incredulous laughter. The very phrases were worn so threadbare [...]." Nick feels as though he is "skimming hastily through a dozen magazines." He judges Gatsby here, believing him to be lying, concocting this tragic and storied past in order to impress (and, in many ways, he is).