Smell and touch the leather of the bag, it should smell a genuine leather (this is a strong smell). Leather also should be glossy.
Check the brand tag of the bag it has only two colors either gold or silver. Brand tag are either on the outside or inside of the bag.
lets face it nine west bag are not like Michael Kors or Kate Spade bags. They sell Nine West much affordable than the brand I mentioned, you will distinctly know that the Nine West bag is original or not.