In Steinbeck's Of Mice and Men, what further details do we learn about the incident in Weed? How do we find out?

The first information about the incident in Weed comes out in Chapter One when George's suppressed anger at Lennie explodes and he tells him--and the reader--everything he feels about their relationship in a page-long tirade:

"Jus' wanted to feel that girl's dress--'jus wanted to pet it like it was a mouse--Well, how the hell did she know you jus' wanted to feel her dress? She jerks back and you hold on like it was a mouse. She yells and we got to hide in a irrigation ditch all day with guys lookin' for us, and we got to sneak out in the dark and get outta the country."

Then, in Chapter Three, George tells Slim about the Weed incident, and we readers get a few further details. George was not with Lennie when the incident started. He came running but had to hit Lennie over the head with a fence picket before he could make him let go. George explains that Lennie became confused and "scairt" when the girl started screaming. 

"So he reaches out to feel this red dress an' the girl lets out a squawk, and that gets Lennie all mixed up, and he holds on 'cause that's the only thing he can think to do."

The Weed incident foreshadows what will happen in the barn when Lennie can't or won't let go of Curley's wife's soft hair. Only, this time, George is not present to interfere. What happens to Curley's wife in the barn might have happened to the girl in Weed right on the main street of town if George hadn't been there to force Lennie to let go of her red dress. 

When George sees the dead body of Curley's wife in the barn, he realizes that he can't trust Lennie out of his sight anymore.

"I should of knew," George said hopelessly. "I guess maybe way back in my head I did."

The word "hopelessly" tells how George feels. He is worn out with looking after this giant with a child's brain. We do not understand how he resolves his inner conflict until the last chapter, when it turns out that it was he who took Carlson's Luger, and not Lennie, as everybody else assumes. We realize at this point that George had made a decision to kill his friend rather than let him be tortured and killed by Curley's lynch mob.