Usually the expensive sunglasses with big brand will have the special print on the frame. You can firstly check its brand by the comparison with the regular ones from the anti-fake label. Then you can check the manual work of the sunglasses to see whether it is smooth in lenses and frames. If you still have the doubt, you can go to the store and ask the client.
I never buy dolce and gabbanas sunglasses because they are expensive. If you want to buy such brands sunglasses, you can go their exclusive shop. I believe you will get real dolce and gabbanas sunglasses there. Besides, you shall check their sunglasses frames and lenses. You know, Famous brands sunglasses always printed with their unique logos on their products. Good luck.
I will recommend you several ways to judge it. First, you can view the manufacture place of origin, pay attention to the production time. Second, you should compare the prices of sunglasses in multiple stores, if the price differences are bigger, you should consider there may have fake sunglasses among these sunglasses. Third, gradient is the color of the lens is changed from one color to another color, and it can block the glare of the sun. So you can judge the sunglasses thorough this aspect. Fourth, sunglasses have another function, it can reflect light, and people can not see your eyes when you wear the sunglasses, but it can make you clearly see the scene outside. Hope these methods can help you.