maybe check about this mac, system report, usb, it should say
"apple watch magnetic charging cable", and it would tell you it supports 1000 ma ( current available 1000, current required 500, extra operating current 500 ma. and give you the apple product ID and vendor ID numbers
thats the only thing i can think of
if system report says its not an apple one or its only capable of 500 ma, then its a dead giveaway that its a fake one. i think recent macs running el capitan should give you the correct charging currents.
whats different between stainless steel and sport charging cables?
i wouldn't know how to know if one is legitimate. system report is the only thing i could think of