A sentence to explain. (Revised)

How could I know what she was capable of? I'd seen what regular ordinary men could do. I'd seen newsreels of what they found after the war.


This is a sentence in a novel.
A 16-year-old girl wonders whether her mother is a murderer.
Before the sentence she says,

No If I knew one thing, I knew that Mom didn't do it. She put her hands over her ears during thunderstorms.
The astray had flown through the air and shattered. Her face had been so blank.
(BTW, in the previous part of the story, her mother had thrown an astray towards them. And the story happens in 1947 when WWⅡhas just ended. )