I was given a pair of Gucci sungles. how do i know if they're legit? brand new never been used with a tag!!?

first and biggest clue, where were they made? if in China then they are absolutely fake, made in Italy, they are most likely real (and the made in country cant be "fake" it's mandatory on all products) second, look fo serial numbers, they should be very small and printed on the inside of one of the lenses.