As the heroic character of Steinbeck's Of Mice and Men, Slim is described in Chapter 2 as a tall man with an ageless hatchet face.
...and he moved with a majesty only achieved by royalty and master craftsmen. He was a jerkline skinner, the prince of the ranch, capable of driving ten, sixteen, even twenty mules with a single line to the leaders....There was a gravity in his manner and a quiet so profound that all talk stopped when he spoke. His authority was so great that his word was taken on any subject, be it politics or love.
That Slim, with his "God-like eyes" that are "level and unwinking," and ears that hear more than is said and gentle voice is respected is evinced in the way in which all the men speak to him. The first to mention Slim is Candy as he welcomes George and Lennie to the ranch in Chapter 2:
"....Slim's a jerline skinner. Hell of a nice fella. Slim don't need to wear no high-heeled boots on a grain team."
Then, in this same chapter, Curley's wife stands in the doorway and speakes to Slim, who kindly says, "Hi, Goodlookin'." But, when Curley's wife pretends that she is trying to find Curley, Slim retorts,
"Well, you ain't tryin' very hard. I seen him goin' in your house."
She was suddenly apprehensive. "'Bye boys," she called into the bunk house, and she hurried away.
This passage indicates that Curley's wife respects and fears Slim because she does not tarry in the doorway as usual; instead, she scurries away. After Curley's wife leaves, Carlson enters the bunkhouse and immediately speaks to Slim, showing his respect. Then he broaches the subject of Candy's dog, but only after first asking about Slim's pups,
"Why'n't you get Candy to shoot his old dog and give him one of the pups to raise up?"
But, the dinner triangle rings before Slim can respond. He tells the others to get something to eat. Here Steinbeck writes,
"Carlson stepped back to let Slim precede him, and the the two of them went out the door." 
Also, in Chapter 2, after Slim enters the bunkhouse and meets George and Lennie he looks at them with kindness and speaks gently, "Hope you get on my team." In Chapter 3, they converse and Slim easily wins George's confidence. As George's voice takes on a "tone of confession" to Slim, he trustingly says before revealing what happened to Lennie in Weed,
"You wouldn'tell?...No, 'course you wouldn'."
Later in this chapter, as another indication of the respect and recognition of Slim's superiority, Whit asks Slim to read a magazine article about one of the former ranch hands in an effort to attain some approval.
Then, when Carlson is going to shoot Candy's dog, Slim tells him to take a shovel, and he responds, "Oh, sure! I get you." His acknowledgement is a manner of respect. Later, when Slim when Crooks, the stable buck, sticks his head into the doorway, he is extremely respectful as he tells Slim that he has prepared the tar for a mule's foot. When Slim says that he will be right out, Crooks offers to do the job for him addressing him as mister, "I can do it if you want, Mr. Slim.
Clearly, the men perceive Slim as their boss more than they do Curley or his father. For, it is with great respect that the ranch hands speak and react to him. In the conclusion of the novella, no one says a word when Slim tells George, "You hadda." This is the final say on George's shooting of Lennie.