Good information on How to Spot Fake Memory Cards (CF and SD) from the below website.
As fake memory cards have now proliferated throughout the marketplace, it is important to be able to spot a fake. This page includes methods to spot fakes, and examples of fake cards.
Why should you care?
Inferior quality card - youíre not getting what you paid for
No warranty - when the card fails, youíre out of luck
Fake cards sometimes have invalid partition sizes, making it likely to fail
It will drive costs of genuine cards up
Spotting a fake
Unfortunately there is no software which will test the card, and ensure that itís genuine. You must manually scrutinise your card, (both visually and by testing itís performance), to get some idea of itís genuinity.
Presentation quality and differences
First thing to look for is poorly printed labels and material; most fake cards have slight differences from the real deal. Most manufacturers have a page where you can download high resolution pictures of their cards. Below is a list of manufacturer image page links: