You can distinguish them first by price. Real authorized retailers are not allowed to deeply discount their products . This means that if the retail price is $200, they may be able to sell them at $180 . They are not ;however, able to sell them for $120.
If you are going to buy Rayban sunglasses online, always look at the "about us" and " Contact us" section of the web sites. Check the telephone number and address to see if they are in US. If they are in China or other country, they are absolutely fake! But being in the US alone does not mean buying authentic, being abroad and overseas usually mean buying fake!
Ray Ban sunglasses are always expensive. When you choose Ray Ban sunglasses, you should be careful with the logo. Real Ray Ban sunglasses usually have the BL symbol etched on the left lens, which is next to the hinge while fake Ray Ban sunglasses's logo may be located on other places.