Most of us have heard tales of cheap memory cards not being genuine or presenting themselves as having a larger capacity than they actually have. There are many dealers on the net selling fake or mis-represented cards.
I'm currently looking for an SD card, not for my 580 but for an e-book reader and came across this: H2testw v1.4.
What is it? basically it is a freeware programme that checks your memory cards and gives you an idea of whether they are genuine or not.
If you are unlucky enough to buy a fake this might give you a bit of leverage when trying for a refund.
Hope this helps someone.
I did mean counterfeit cards and had originally intended to add 'caveat emptor.'
Another well used phrase is: "if it looks too good to be true, it generally is."