I bought a digital camera that came with a 4GB SanDisk card off Craigslist. The guy said he purchased it from radioshack. How can you tell if it's a fake? I did all my research for a Sony but it's a Sandisk and have absolutely no idea on how to tell if it's a fake. Any testing I can do? It's a blue card and on the back it says made in china and has a line of numbers. Thanks.