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.
With regard to fake Sandisk cards, I notice that on older cards, the Sandisk Registered logo was molded into the back face of the card, (this is quite difficult to fake) however, on the new cards, this does not appear to be so, both the old and new, had (made in china) on them, I therefore wonder if sandisk would comment on why they have discontinued this practice, or whether ALL cards without this logo are fakes ?
With regard to fake Sandisk cards, I notice that on older cards, the Sandisk Registered logo was molded into the back face of the card, (this is quite difficult to fake) however, on the new cards, this does not appear to be so, both the old and new, had (made in china) on them
Don’t know how “old” you’re referring to, but I have numerous SanDisk SD cards, the newest being only a few months old and going back in age to about 5 years.
None of them have any registered SanDisk logo (or anything for that matter) molded into either the front nor the back of the card.