If you’re trying to figure out whether someone has a fake smile, look at their eyes. When you have a genuine smile, the corners of your mouth upturn, your cheeks raise, and the skin around your eyes crinkles. Known as the ‘Duchenne smile’, it happens involuntarily when you’re truly happy about something- so a smile without eye crinkles is a good indicator that it’s forced or fake. Source Source 2 Source 3