The fake von zipper sunglasses must be with the unsmooth surface which will be felt easily by the hands. The fake ones will have no exact brand print. You can check it according to the exact ones given online. Then you can put it on the table to observe its balance. The fake ones must not be in symmetry.
Sometimes, it is really difficult to spot which one is fake von zipper sunglasses. To get real one, you shall buy von zipper sunglasses in their retail store. I know they have a store in the Seattle, WA. Besides, they also sell the sunglasses online. Hope you can find their sites. Anyway, here i find a video which teach you to spot fake von zipper sunglasses at https://www.youtube.com/watch?v=1FZA_WXPDHY. Good luck.