In "Lamb to the Slaughter," when does it become obvious that Mary is going to kill her husband?

I do not think that it ever becomes obvious to a reader that Mary is going to kill her husband, Patrick. The first time that I read the story I was shocked and surprised that Mary killed Patrick.  My students are always shocked by the event as well.  I don't believe that Dahl gives any strong indication that Mary is thinking about killing her husband.  In fact, I don't think Dahl gives readers any indication that Mary is even capable of killing Patrick.  I believe that Dahl intended for that moment in the story to be completely shocking.  The beginning of the story paints Mary as the perfect doting wife that lives to be in the glorious presence of her husband. 

She loved to luxuriate in the presence of this man, and to feel almost as a sunbather feels the sun that warm male glow that came out of him to her when they were alone together.

Readers are shown Mary being a person that simply can't function or live without Patrick.  The thought that she would kill him is unbelievable.  I can picture Mary grovelling at Patrick's feet and begging him to change his mind.  I can picture her going into a catatonic state too.  Those would be obvious reactions for Mary.  I do not believe that Mary's intention to kill Patrick is ever obvious, and if I'm honest, I don't think that Mary ever intended to kill Patrick either.  I think that she wanted to physically hurt him, and that is why she swung the leg of lamb at him.  Unfortunately, Patrick dies.  That forces Mary to make an intentional decision to get away with it.  

As the wife of a detective, she knew quite well what the penalty would be. That was fine. It made no difference to her. In fact, it would be a relief. On the other hand, what about the child? What were the laws about murderers with unborn children? Did they kill then both-mother and child? Or did they wait until the tenth month? What did they do?
Mary Maloney didn’t know. And she certainly wasn’t prepared to take a chance.

It's at that moment that readers can be quite certain that Mary is going to try and cover up her involvement with Patrick's death.