Eyes. You can always spot a fake smile. Usually when people honestly smile their eyes squish up a bit more, and their cheeks.. .too.. But in a fake smile the face is more "perfect" as in the cheeks and eyes aren't all squished up. I've also noticed that in more fake smiles the eyes tend to stay larger instead of squitning a bit. As well as the eyebrows.
Hey..when you are a cheerleader, you gotta learn to fake a pretty good smile.