Here are a few pointers that I found in an eBay guide:
1. The "sheen" on the FAKE bags are extremely shiny.
2. The interior of the FAKE bags are either made of cheap yellow felt or white cotton that is supposed to look like the authentic combination of linen/cotton and hemp interior.
3. Stitching on FAKE Goyards match the color of the vinyl/pleather. Authentic Goyard totes have white stitching at the top of the bag and will have a white stitch in the middle at the end of the strap that is attached to the body of the bag. Also, many of the FAKES have crooked straps.
4. Don't be fooled by the FAKE Goyard tags dangling off the bags and the fake dust covers.