How can I tell if a PS3 controller may be fake?

I believe that the pictured controller is fake. There is a forum post on the PS community forums about this with a very long and detailed list of differences.

From the pictures you have posted the rear section with the screw, label and reset button are what make me suspect that this controller is not genuine.

The picture below is from the link above and is of a fake:

This looks very similar to the one in your question, its hard to tell in you photo but one way to be certain is that there is no reset button, only a hole in the case.

Here is a picture of a real controller for comparison:

The second thing that makes me suspect that it is a fake is the manual, below is a picture of the fake manual on the right and real manual on the left:

There are many other subtle differences to look out for that are not clear from your photographs, my advice is don't buy if you have any doubts.

Turns out it was a fake. I have returned the controller.

Analog sticks were smaller and had only 8 directions.There was also no reset button as you predicted.

I'm looking for original controllers as well and I can tell the ones in these pictures are fake. Today I disassembled one I know being genuine and one I bought with the same packaging as the picture above. The genuine one comes with a Toshiba chipset and a good weight battery. The second is indeed a cheap counterfeit with one chipset covered with resin, and a battery half the weight of the original one. Also worth noting that R2 and L2 on the fake controller are just buttons, not analog. Both shells are different and not exchangeable. Re-assembling the genuine controller was easy and straight forward while reassembling the fake one proved itself to be a nightmare.

It becomes obvious while playing with the fake one, that the left analog, in fact, reacts almost as bad as an 8 ways joystick, making characters take stiff angles while running in a circle.