First giveaway is the box itself. If you bought a casing for S4, the box should clearly say Samsung Galaxy S4 on the front box unlike the fake box on the right which doesn't mention which phone model.


Turning back the box, if the casing is for a Samsung, it should state Samsung's copyright near bottom left, unlike the fake box on the right which has Apple's copyright even though the casing is for an S4. 

Plus, check out the example phone image for the box on the right. It's clearly shows an iPhone picture.



Moving on inside the box, the original one will provide a grey cloth (unseen here) for the screen protector while the fake one is provided with a yellow cloth.

Do also notice the screen protectors has different cutouts for the home button at the bottom.



The little feet that supports the casing at the back for the original casing is also thicker than the fake one which is almost insignificant.



For the S4 casing, there's a little feature between the camera and flash hole which the fake one lacks of. The overall finishing of the original one should be smooth while the fake one looks messy (dried glue appearing at some places, unsmooth surfaces).