Van Cleef & Arpels is counterfeited almost entirely through one design: the Alhambra clover, produced in enormous volume by the maison and in greater volume by everyone else. Its authentication is unusually specific, because the motif has countable features.
Each Alhambra motif is edged with a row of gold beads, and on genuine production those beads are uniform in size, evenly spaced, and consistent in count around the clover. Counterfeits vary the bead size, space them unevenly, and frequently miscount. It is a check that takes a loupe and a minute.
Beyond that, the maison's hallmarking is French and rigorous — assay marks, the maison's poinçon and a serial — and its stone setting is done to a standard that a counterfeit does not reach.