When she was shown the scene of the crime, Detective West realized something was missing. Jack Watkins was preparing to go to a formal dinner, but nowhere in the room were the white dress shirt, bow tie, or studs he would need for his tuxedo.
Then she remembered the smear on Watkins’ hand. Nothing in the room was found that indicated where he had wiped the hand. She realized that after he was shot, Watkins must have clutched at his wound, getting his hand bloody. Then he wiped the hand on the closest thing he could– his killer’s clothing.
The killer certainly couldn’t leave Watkins’ room with bloody clothes, so he removed them, burned them in the fireplace, and put on Watkins’ shirt, tie, and studs.
The only person in the group in a tux was Terry Jones, so West deduced he was the killer.