An apparatus and a method for detecting the material of a surface of flat objects on a stack, in particular of objects individually separated from one another in the stack by interlayers, preferably of printing plates. The object is to reliably detect the material of the surface of the object that was taken from the stack before it is provided for further processing. The object is achieved by a sensor apparatus that carries out a measurement of the electrical resistance in the region of the object surface to be determined. For this purpose, contact is made between the surface and sensor electrodes, and a measuring current is conducted through the surface. On the basis of the current intensity determined, a distinction is made as to the material of which the object surface is formed.