Mary shows her observant nature throughout the entire storyline. We see it first in how she notices that her husband is behaving strangely; he chugs his entire cup of whiskey. She is so observant that Dahl writes:
"She wasn’t really watching him, but she knew what he had done because she heard the ice cubes falling back against the bottom of the empty glass when he lowered his arm."
Not even looking at her husband, she knows he has downed his drink, which, she notes, is "unusual." She continues to watch him through the conversation, becoming more alarmed.
Her observant nature continues until the end of the story. Even as she is sobbing out her story about coming home to discover her husband dead, she takes note of what the detectives say about what the grocer said. Dahl states that despite "her sobbing she heard a few of the whispered phrases" of the detectives, confirming her alibi and whether they believe it. As the search for the weapon commences, she sits in the front room, but listens carefully, and can tell that there were cops everywhere because she could "hear their footsteps." She even notices as it gets late that the policemen were "growing weary, and a trifle exasperated." And, as they eat, she listens to their words. So, she is very observant, from beginning to end.
Her calculating nature comes out more after she commits the crime. She immediately concocts a plan to cover her tracks, brings the cops into the house, listens to them carefully, and then when they are getting restless, she very carefully manipulates them into taking a drink, and eating the lamb. Her calculating up through the end of the story works out perfectly, and it is the cops themselves that eat the evidence of the crime.
I hope that those thoughts help; good luck!