How can you tell that Patrick is preoccupied in "Lamb to the Slaughter"?

Since Patrick is killed fairly early in the story, the reader does not get a lot of time to figure him out. Despite that disadvantage, Dahl writes the character in such a way that the reader immediately does not like him. Part of the reason that the reader feels distanced from Patrick is that he is cold toward Mary and seems preoccupied the entire time.  

Immediately on meeting Patrick, it seems like something is on his mind. Dahl has spent multiple paragraphs describing how Mary anxiously awaits the moment that Patrick walks through the door. When he finally comes home, Mary goes to him, takes his coat, and gives him the "hello darling" welcome. His response . . . "Hello." It feels flat, distant, and cold compared to Mary. 

A few minutes later, Patrick gives another clue that his mind is preoccupied.

And as he spoke, he did an unusual thing. He lifted his glass and drank it down in one swallow although there was still half of it left. He got up and went slowly to get himself another drink.

Based on those lines of text, it seems that Patrick would usually savor his after-work drink. Not this time. He drinks it all in a single gulp and gets another drink. Either he had a terrible day that he is trying to forget, or he is trying to find some liquid courage. It turns out to be the latter, but it is clear that something is bothering Patrick. 

The last evidence to support his mind's preoccupied state occurs when Mary tries to engage him in small talk.

"I think it's a shame," she said, "that when someone's been a policeman as long as you have, he still has to walk around all day long." He didn't answer. "Darling," she said, "If you're too tired to eat out tonight, as we had planned, I can fix you something. There's plenty of meat and stuff in the freezer." Her eyes waited to an answer, a smile, a nod, but he made no sign.

Mary's talking, but Patrick isn't responding. He's either ignoring her intentionally, or he isn't really even listening to her. I think he's so focused on the news that he is about to break that he doesn't hear Mary.