The Fendi Zucca print is having a real resurgence of popularity at present but like all of the luxury branded bags Fendi are copied far and wide and you have to have a keen eye to spot the difference.
Post 2004 Fendi introduced the hologram sticker for bags. These stickers are extremely detailed and are found on the fabric tag inside the bag. The reverse of the tag will also have an alpha numerical serial number. If the bag has neither of these then leave it alone! Beware however, as the fakes also have holograms but these will peel easily whereas the real deal is VERY firmly fixed!
However if you are looking to buy a vintage Fendi they will have neither of these and you will need to rely on knowing your way around the logo and hardware. On bags such as the Mama and the Baguette study the FF hardware to the front. The Fendi branding will be engraved to the side and to the back.
Remember the devil is always in the detail. Make sure that there are no mis sized stitches or inferior zippers. Luxury bags are just that and should be made to the highest standard with the highest quality materials.
The buckles on Fendi bag straps are perfectly square and will have fendi engraved to the side and zippers will have the FF branding to the underneath.
Websites such as the Purse Forum are great places for further reading on authenticity for specific Fendi models.