The present invention relates to devices and methods for inspecting filters for extracorporeal blood treatment devices, e.g., filters for filtering dialysis fluid. The inspection of the filter according to the present invention is based on the measurement of the flow potential before and after changing the fluid flow of an electrolytic fluid between a transversal flow through the semi-permeable membrane of the filter and a longitudinal flow along the semi-permeable membrane of the filter, or vice versa. Changing the flow direction of the electrolytic fluid results in a change to the conductivity proceeding from a base value P to a higher value P2 or a lower value P1, wherein the conductivity readjusts to the base value P after a certain time interval T1, T2. An improper state of the filter is determined on the basis of the change in conductivity after changing the flow direction of the electrolytic fluid.