Every pigment or eyeshadow should have a shade name, NOT a number..ever!! However they will have a batch code, such as A28 aswell as the name.
(you can always verify it is a genuine MAC colour by googling "MAC Helium"etc just to double check)
You can pretty much always tell fake items, by other things they are selling. For instance, if you look at the eyeshadows they are also selling..You can tell they are very very obviously fake. They are round & shiny. Chances are if they are selling fake eyeshadows/lipsticks the pigments are 99% a fake. It just helps, because its hard to tell with pigment jars.