He introduces Curley as a thin young man then begins to describe his body language, which is instantly aggressive and threatening:
"...hands closed into fists"
"...slight crouch. His glance at once calculating and pugnacious."
Lennie's reaction gives us an indication too, as he shrinks away from Curley and is instantly nervous.
Curley goes on with a more and more aggressive tone:
"Let the big guy talk."
"By Christ, he's gotta talk when he's spoke to"
"Oh, so it's that way"
This is the first time he's met either Lennie or George, so the reader gets an instant disliking to Curley, recognizes him as the antagonist, and is prepared to see a future confrontation in the story.
In Chapter Two of John Steinbeck's Of Mice and Men, when Curley first enters the bunkhouse, he wears a work glove on his left hand, and high-heeled boots. As mentioned above, he is a small man, so the boots suggest that he wishes to appear taller and more imposing. Then, as he eyes "passed over the new men," he stops. "He glanced coldly at George and then at Lennie." As soon as he looks at George and Lennie, he goes into a fighter's stance:
His arms gradually bent at the elbows and his hands closed into fists. He stiffened and went into a slight crouch. His glance was at once calculating and pugnacious.
And, when Lennie squirms under his look and shifts his feet nervously, Curley sees his intimidated behavior and "stepped gingerly close to him" sensing his advantage, as a fighter would. Then, when Curley speaks to Lennie, George answers, holding to diffuse the situation. However, Curley becomes angry and "lashed his body around, yelling at George to let the "big guy talk" as though he wants Lennie to "start something." As he turns to leave, he tells Lennie, "Well, nex' time you answer when you're spoke to"; turning, Curley leaves with his elbows still bent in a combative pose.
After he leaves, Candy tells George that Curley is "like a lot of little guys. He hates big guys" and tries to prove that he is tough by fighting them. The old swamper describes Curley as "handy":
'Never did seem right to me. S'pose Curley jumps a big guy an' licks him. Ever'body says what a game guy Curley is. And s'pose he does the same thing and gets licked. Then ever'body says the big guy oughtta pick somebody his own size, and maybe they gang up on the big guy. Never did seem right to me. Seems like Curley ain't givin' nobody a chance."