(57) In order to establish that written matter, a drawing, a document, or the like, being present in the shape of sheet-formed material, has been copied, said sheet-formed material is provided with a marking. Said marking consists of a substance which in ordinary environmental conditions is either invisible or has a special appearance. When exposed to strong light, e.g. a flash lamp, or heat of more than 40°C said substance becomes permanently visible or changes its appearance, respectively. On an original document it will, thus, be possible to detect that the docu­ment was, e.g. put through a copier. Said sheet-formed material may be completely impregnated with said substance or it may be provided with said substance in the shape of a pattern, if desired, such marking may be provided on an ordinary sheet.