Visit Gucci's website
Gucci has an interactive website that displays all its products. Prior to shopping for sun glasses, you should visit it to educate yourself on the kind of sunglasses that are available. You should also know about all the relevant information and price range that they're available at.
Check the tag
Gucci products have a 'Made in Italy' on its tag. This is another proof of their authenticity and any sunglasses that say, "Made in China" or, "Made in Taiwan" will obviously be fake.
Check the spellings
Gucci is a brand that never compromises on quality. This includes making any spelling mistakes too. When you're buying sun glasses, make sure there are no spell errors. Replicas will usually have spelling mistakes or will have Gucci spelt differently to avoid copyright claims by the company.
Gucci sunglasses come with a warranty card!
Since Gucci is a brand that believes in after sales services, therefore their sunglasses also come with a warranty card and information booklet. If the glasses you're looking at have none of those, they are probably fake!
The quality speaks for itself!
Original Gucci glasses will be of high quality, speaking for the brand's repute. On the other hand, replicas will have loose screws or the glasses/plastic in them will be of low quality.
Gucci has a logo - and it is for a reason!
A logo is part of a brand's image, and it can be seen on all of its products. The difference between a fake and real Gucci logo can be clearly seen in the picture on the right side. Also, Gucci sunglasses will ALWAYS have a model number on them. Tally it with the ones available on the website, and you're sure to buy original sunglasses.