The best way to tell if a SD card is fake is to run the H2 burn in test. However, you do need to have the card in your possession to do so, but I am sure if you are unsatisfied or can prove that the SD card is fake, the seller will have to reimburse you. For more info on testing for fake flash memory, check out my page which has a tutorial with video guide.
http://www.chinaelectronicsinsider.com/technical/how-test-fake-flash-memory
