Just make sure not to use the mouth as your only thermometer: “It is the eyes that will give away a person’s true feelings while they are smiling,” says Dr. Adam Shulhof of the Schulhof Center for Cosmetic Orthodontics. “This is because when a person is truly happy and smiling a genuine smile, the cheeks should rise, resulting in the skin around the eye region crinkling.” Reading the eyes, the musculature around the face, and the shape of the mouth together can clue you into whether someone is really happy or faking it.