You use CrystalDiskMark to benchmark it. Visual inspection as well. Most real cards are made in China, knockoffs are made in Malaysia but its not always the case. Counterfeit will be 1/3rd of the speed of regular one and most likely much shorter shelf life since its pretty much old generation chips used.
I build custom video setups as hobby so I say overall probably I got over 150 cards from Amazon (at least half were fake) and some 100 from Newegg so far (last 3 years) and haven't had a single fake from Newegg. [mostly I use Sandisk and Lennar cards]
