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Made from real calf leather, Prada leather bags should be supple and smooth. If the leather is stiff or unbending, it's likely a fake. The interior will either be embossed jacquard nylon fabric or nappa leather. The fabric will have a pattern that alternates a line printed with Prada and a line of rope motif. Every other line with the Prada logo will be printed upside down. Real Prada bags should not have stitching that is crooked, uneven, or sloppy. The stitches will be small and sturdy. If they're frayed in any areas, that's a sure sign the bag is a fake. On a leather bag, the stitches should match the color of the leather. Designer bags almost never have seams that are glued.
Run your hand over a leather bag to see if it's soft to the touch. Check that the interior fabric is high quality and has the Prada pattern. Look for clean stitching along the seams.