How do you know Huck and Jim are friends by the end of Chapter 11?
By the end of Chapter 11, it is quite clear that Huck and Jim are friends (at least from Huck's perspective -- we do not really know yet what is in Jim's mind in this regard). We can see this quite clearly because of how Huck has come to be protective of Jim.
Here are two examples from Chapters 10 and 11.
- When Huck causes Jim to get bitten by the snake, he does not admit he did it. This may not sound friendly, but it shows Huck's concern for Jim's feelings. He knows Jim's feelings will be hurt if he finds out Huck's prank was what caused him to be bitten. Huck does not want that and so he stays silent.
- Huck hurries back from the town to warn Jim and to get away when he finds out Jim is suspected of killing him. Moreover, he doesn't even think of saying that the people are after Jim -- he thinks they are after both Jim and himself. This shows he is identifying himself with Jim just as a friend would.