You're the Detective: 2. Cheaters Never Proper

Solve the case before looking at the solution at the end of the story. You are the Detective.

Cheaters Never Prosper

"Life's a garden  -- dig it."
    -Joe Dirt
   
    On the foggy rainy night of June 4, 1979, the Smith family was having a party.
    “Can you help zip my dress, Honey?” asked Mrs. Smith.
    “Sure thing, dear,” replied Mr. Smith.
    The Smiths were the average family who lived in a big house with lots of trap doors.  Mr. Smith had one eye. The two never fought and they always got along well, except for Forrest Smith. He had always had a thing for Mrs. Smith, but ever since first grade, she had liked Mr. Smith. 
    “Everyone to the dining room!” yelled Mr. Smith.
    When everyone was at the table, they noticed Forrest was gone and so was Mrs. Smith.  “Where are those two?” asked one of the guests.
    “Well, my wife is probably getting the food ready, but I don't know where Forrest is,” said Mr. Smith.
    Suddenly they heard a gunshot and they all rushed up stairs to find Forrest Smith lying dead on the floor of Mr.Smith’s book room.
    “What happened?” said Mrs. Smith as she ran into the room.
    “I don't know; we were all sitting at the dinner table, except you, and him.  We heard a gunshot and ran up here as soon as we could,” replied Mrs. Smith.
    “I was in my room buttoning my sweatshirt when I heard the shot. I immediately called the police,” said Mrs. Smith.
    The police came in and started chalking and investigating the crime scene. “Come with me, Mrs. Smith.  I will question you first,” the detective ordered. “Tell me what exactly what you were doing at the time you heard the gunshot,” asked the detective.
    “Well, I was in my room buttoning my sweatshirt when I heard the shot and immediately I called you guys,”
    "Thank you, that will be all for now. Will you please tell your husband to come in here, please?” asked the detective.
    “So, Mr. Smith, tell me what were you doing at the time you heard the gunshot?” demanded the detective.
    “I was at the dinner table with all the other guests, but Forrest and my wife weren’t there. I thought she was preparing the dinner,” said Mr. Smith.
    “Okay, thank you, Mr. Smith.  I now know who did it,” replied the detective in a very cheery way. ( by Glenn)

How did the detective know who did it? (Solution follows at the bottom).
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He knew because Mrs. Smith said she was in the other room buttoning her sweatshirt, but actually she had a dress on.