There are a lot of memory cards/ usb drives out there are fakes. These usually are the bigger capacity 1-4gb memory cards/drives. They're usually reprogrammed chips so that it would reflect a larger capacity when in fact these are only 256mb or 512mb or 128mb. The point is that you're paying for something that you can't use. One way to check is to copy and fill your flash drive/cards to its advertised capacity and then read from the drive back. Ex: if you have 1GB sd card, try to copy files as big as 1GB then transfer that back from the sd card and see if you can get it done. Usually fake cards takes forever to copy and fill the entire card.
Here's a sure way to know if the kingston cards are genuine or fake:
http://www.kingston.com/asia/verifyflash/
Just follow what is needed and you're good to go.
I'm sure other brands also has a way of knowing it but I only have first hand experience with kingston cards.