Genuine bumpers are heavier than copies. Also they may not have the foglamp holes cut into them, that is up to you to do if you need foglamps or not. To be able to tell whether its a genuine article or not, look on top of the bumper skin and there should be a tab running along the legnth of the bumper where a seal is fitted. This seal covers the gap between the top of the bumper and the metal colour coded strip that is attached to the grille. If there is no tab, its a copy. If there is a tab, its either a good copy or an original.
But as already said, there should be a VW insignia stamped or moulded onto the back of the bumper skin behind the bumper frame.