Grayson gripes about the money he claims George ran off with as being $50,000 several times, but erroneously says it's $70K in one scene.
When Brooks Mason and George Smith (both played by Robert Young) shake hands, George leans forward slightly, revealing a misalignment between George's right arm and his shoulder. That's the only reveal of the otherwise undetectable trick photography.
Cecilia tells Brooks that she wishes he would look at her "like that", when the Hawaiian girl (Eleanor Powell) is done dancing. But all the time Cecilia was staring at him watching the dancer, he was sitting with his back turned to her, so there's no way she could have seen how he watched the dancer.