First of all, you should put it on the even place to observe its balance. Usually the fake ones with bad manual work may not be balanced on two sides. Secondly, the fake ones may lack the brand name on the frame side. You should observe it. Last of all, you can even observe the detailed manual work of the sunglasses to whether it is made smoothly with no flaw.
Ok, I can see that you really wanna get one pair of it and really hate a fake one. So, to do that, you should bear in mind several things which would help you succeed ok? Well, as you can see, Locs sunglasses are not very cheap so you gotta make a big budget before going to the store. Also, you should go to buy them yourself since we can still find fake ones frequently on the Internet. At last, when you have got what you want, you should check its quality carefully and make sure all the certificates and lables are there. Good luck!