Firstly, I'm not saying all sold on Amazon are fake. If you buy DIRECT FROM AMAZON chances are your item will be genuine, otherwise, I wouldn't trust any other seller. I, like many others in the reviews on here have been hood-winked into buying an Official Sony product that turns out to be a cheap fake replica from Asia (I'm aware the genuine items are also made there).
When you receive your product, check the packaging- Sony now use a black cardboard angular box with a clear from window, or a blue cardboard box.
I think the US design may now be in what's sometimes called a 'blister pack' style, plastic formed over the pads shape, with card surrounding it.
Check the model number/serial number and the barcode to the Official controller you got with the system. As far as I'm aware I've had numerous official pads that use either the serial number (CECHZC2U) which has a very smooth surface to the plastic, or the newest version I bought in the cardboard pack I photographed (CECHZC2E) which has a noticeable rougher 'matte' surface.
ALSO compare the printed silver 'SONY' stamped logo on the centre of the controller, and the Dualshock 3 blue logo on the top-right side of the controller, next to the USB connector.
Finally, some people have had problems with the fakes not charging, or only working when plugged in via USB. If you think your product is fake, return it and REPORT the seller!!
People buy these fakes in bulk, to sell on at a profit, on sites such as Amazon and Especially eBay!! The Official items come in various colours, I've seen the fakes sold in White (which mine was) Red, Blue, Pink, Yellow.
I've not seen a fake pad in the Jungle Green colour (I had an official pad with a Killzone 3 Boxset) or in the transparent Cosmic Blue & Red colours which came with Infamous 2. Sony also recently released a transparent 'Slate Grey' colour.
It can all get a little confusing as there are so many versions now, but B=be sure to check yours carefully when you receive it & before you use it, don't get ripped off while filling some con-artist's pocket! Thanks for reading.