Check the inside tag (genuine tags are heat-pressed and crisp), the fabric name on the tag (it must match the style), logo placement and size, stitching quality, and hardware such as logo-engraved zipper pulls on Define and Scuba pieces.
Counterfeits often get fabric wrong: an 'Align' in slippery, shiny fabric is not Nulu. Our guide on spotting fakes covers each major style.