The first thing you should keep in mind while buying anything is the price. Since Gucci goggles are known for being expensive, so if you are being offered a low price then there is no need to buy them. Keep in mind that Gucci goggles are for no less than $150 to $350; which is one sign of originality.
Upon carefully examining the stripes of the bands, you will notice that the fake one will be faded in color. Besides, you will see "inspired by" on the logo which may appear differently on different goggles.
Upon examining the lenses carefully, you will notice that if the goggles are real then they will be light and clear as crystal. Besides they will also provide complete protection from UVA/UVB harmful rays.
You should also check the model number online for further confirmation of the authenticity.