A Gucci bag is authentic if it comes with a controllato card, if it comes with a dust bag that has the double "GG" or "Gucci" name written in gold across the front and if it has a leather tab inside the bag with a serial number and the words "Gucci Made in Italy" written on it. Fake Gucci bags will be significantly cheaper than real bags and will have minor variances, such as uneven stitching or different fonts on the logos.
When buying a secondhand Gucci bag in person or from sites like eBay, it is important to make sure that the bag is in fact an authentic Gucci before a purchase is made. When buying online, make sure that there are close-up views of the logo and stitching. If the logo does not match the Gucci logo exactly, then it is a sign that the bag could be counterfeit. Similarly, if the stitching is uneven along the edges, or if the bag does not come with a controllato "checked" card, then there is a high chance that the bag may not be actually made by Gucci.
Gucci does not wrap their bags in plastic, so stay away from bags being sold in sealed plastic. Finally, inspect the inside tab of the bag and the YKK zipper. If the tab does not have a serial number or if the zipper does not match other zipper handles exactly, then the bag is most likely fake.