if you look at the 4 pins the outside pins are the power and the inside pins are the data connections and the power pins got a + and a - side and if the polarity is reversed the flash drive and all other USB equipment sensitive to voltage polarity will be damaged
and if you look nicely, cables are marked "THIS WAY UP"
there is a usb logo on one side of the connector, and nothing on the other side...guess which side should be facing up? (or right if you look at a tower chassis from the back)
if you look at flash drives etc, then one side of the connector has a seam, as mentioned previously...seam is the bottom side.