This is not a visual sign, but if you suspect that you have purchased a fake one, you can try this process.
UD’s website says that each eyeshadow is 0.05 ounces. I recorded the mass of the whole real palette (4.65 oz) and took out one of the eyeshadows (the one that I had used the least so that its mass would be the closest to an unused unit’s mass), and weighed the whole thing again. This removal led to a 0.15oz decrease in mass. Basing from this information, I am assuming that the eyeshadow powder itself is 0.05 oz and that the eyeshadow pot is 0.10 oz.