In To Kill a Mockingbird, when should the reader experience an epiphany regarding Mr. Ewell's left hand?
Atticus establishes during Sheriff Heck Tate's testimony that the bruises on Mayella's face were under her right eye; the sheriff also testifies that there were other bruises found all the way around her neck. When Atticus questions Bob Ewell about his writing hand, Bob signs his name with his left hand. This clearly indicates that it could have been Bob who beat and strangled Mayella. But, as Scout quickly realized,
... Tom Robinson could easily be left-handed, too... I looked down at him. His back was to us, but I could see his broad shoulders and bull-thick neck. He could easily have done it.
The true epiphany concerning the importance of Bob's left hand comes later when Atticus requests Tom to stand and allow Mayella to identify him. Scout had never seen Tom stand before; Tom had been sitting behind a table with Atticus, but when he stood, Scout saw that
His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small, shriveled hand, and from as far away as the balcony I could see that it was no use to him.
"Scout," breathed Jem. "Scout, look! Reverend, he's crippled!"
It was only then that Jem and Scout--and the reader--became aware that Tom could not have physically caused the injuries to Mayella. Her injuries could only have been caused by the left hand of Bob Ewell.
This is important because in proving that Bob Ewell is left handed, Atticus also proves that Mr Ewell was probably the person who beat up Mayella. Bob testifies that Mayella's injuries were on the right side of her face. When you punch with your left-hand, you will most likely hit the right side. Tom could not have done it, because his left hand is crippled.
It is important that Mr. Ewell sign his name with his left hand, because it proves two things. First, that Mr. Ewell is left-handed. The second thing is that Atticus wants to prove that a left-handed man beat Mayella, which is something that Tom couldn't do with his hand being crippled.