An anti-counterfeiting device for application to or association with consumer goods to give an indication of authenticity comprises: a substrate ; a latent image formed on the substrate that becomes visible upon exposure to oxygen; an oxygen-impermeable layer covering the latent image, and means to break the oxygen-impermeable layer. A delta flop is determined by subtracting from the flop at the start from that after the desired time interval and a degree of mottling of a coating resulting from the layer is visually assessed. The projecting portions may have projecting lengths different from each other or may have projecting lengths and projecting thicknesses different from each other.