Usually when gucci glasses are real they will have a number on the inside and be made in Italy. Fake ones don't have a number and are made in China.
Because you will see the either the chanel logo or chanel written near the top on each side of the frame.
1.Check for a warranty manufacture card. Authentic Gucci sunglasses always come with this card and it has the serial number printed on the card.2.Find out where the sunglasses were made. All Gucci...
By looking for any loose threads
If it is fake it will most likely say Gucci on the belt buckle but if its real it will not and the stitching mite look a bit wavey or sloppey