With specific evidence, how dose Christopher know his dad is sad about killing Wellington the dog?

From the text:

At 2:07 a.m. I decided that I wanted a drink of orange squash before I brushed my teeth and got into bed, so I went downstairs to the kitchen. Father was sitting on the sofa watching snooker on the television and drinking scotch. There were tears coming out of his eyes. I asked, “Are you sad about Wellington?”

He looked at me for a long time and sucked air in through his nose. Then he said, “Yes, Christopher, you could say that. You could very well say that.”

I decided to leave him alone because when I am sad I want to be left alone. So I didn’t say anything else. I just went into the kitchen and made my orange squash and took it back upstairs to my room.