Private thoughts are difficult to discern, as they are internal and unspoken. However, there are several clues in the book to make educated guesses about what Crooks is thinking and feeling.
First, Crooks feels alone. We can see this most clearly, when Candy comes into Crooks' room for the first time. To put it into perspective, both men have been on the ranch for a long time, but they never visited each other. Here is the quote:
Candy leaned against the wall beside the broken collar while he scratched the wrist stump. “I been here a long time,” he said. “An’ Crooks been here a long time. This’s the first time I ever been in his room.”
Here is another quote:
Books ain’t no good. A guy needs somebody—to be near him.” He whined, “A guy goes nuts if he ain’t got nobody. Don’t make no difference who the guy is, long’s he’s with you. I tell ya,” he cried, “I tell ya a guy gets too lonely an’ he gets sick.”
Second, Crooks has no hope. When Candy and Lennie talk about their dream about having land, Crooks immediately says that it won't happen. He is very pessimistic. Here is what he says:
Crooks interrupted brutally. “You guys is just kiddin’ yourself. You’ll talk about it a hell of a lot, but you won’t get no land. You’ll be a swamper here till they take you out in a box. Hell, I seen too many guys. Lennie here’ll quit an’ be on the road in two, three weeks. Seems like ever’ guy got land in his head.”
Finally, Crooks enjoys seeing other people suffer on account of how he has been treated. We see this point when he plays with Lennie's mind by insinuating that George might not come back.
Crooks’ face lighted with pleasure in his torture. “Nobody can’t tell what a guy’ll do,” he observed calmly. “Le’s say he wants to come back and can’t. S’pose he gets killed or hurt so he can’t come back.”
This perverse desire is probably rooted in bitterness.
In conclusion, Crooks feels alienated, hopeless, and bitter.