How does Crooks face discrimination throughout the book?

Crooks is rather an isolated character in Of Mice and Men, and his isolation serves to highlight the issue of discrimination. His race sets him apart from the other characters on the ranch, who tend not to spend too much time with him. He's forced to live on his own in a tiny old shack and has the worst job on the ranch, that of stable boy. Simply by virtue of his working and living conditions he's being discriminated against on the grounds of race. He's always defensive around white people. One false word or even a funny look would be enough to get him into serious trouble. When Curley's wife threatens to have him lynched for not treating her with respect, it's no idle threat.
But it's not just the story's unsavory characters who display racial prejudice towards Crooks. George, for example, is less than thrilled at the suggestion of Crooks joining him and Lennie on their dream ranch. It is only really Lennie who sees through Crooks' skin color to see the human being beneath. The fact that it takes someone with mental health issues to do this says a lot about the all-pervading racial prejudice and discrimination existing at that time and in that place.