A subtle design in paper that identifies it as authentic is called a watermark. When you hold a hundred dollar bill up to the light, you can see its watermark.
The term watermark comes from the way these designs are created, during the "wet paper" stage of making paper, and also from the fact that a watermark resembles a damp spot. Documents, stamps, and paper money all have watermarks that label them as genuine. Fancy writing paper often includes watermarks noting the brand. And you can also use the word for a digital marker, a signal that identifies the owner of a photo, video, or audio file. In its most literal sense, watermark means a mark showing the level of a body of water.