A device for checking a filter, which comprises a semipermeable membrane which separates the primary side and secondary side of the filter, characterised in that the device comprises means (20; 25 - 28) for generating a change in the flow direction between a transverse flow of an electrolytic fluid through the semipermeable membrane of the filter and a longitudinal flow of the electrolytic fluid along the semipermeable membrane of the filter or a change between a longitudinal flow of an electrolytic fluid along the semipermeable membrane of the filter and a transverse flow of the electrolytic fluid through the semipermeable membrane of the filter, means (20; 16, 17, 18, 19) for measuring the conductivity of the electrolytic fluid flowing in the transverse direction downstream of the filter and/or of the electrolytic fluid flowing in the longitudinal direction downstream of the filter and a computing and evaluation unit for monitoring the change in the conductivity after the change in the flow direction of the electrolytic fluid from a base value, said unit being configured such that an incorrectly functioning state of the filter is ascertained on the basis of the change in the conductivity after the change in the flow direction of the electrolytic fluid; characterized through a control unit, which is connected with the means for generating a change in the flow direction, the means for measuring the conductivity and the computing and evaluation unit and is configured for performing a method for checking the filter comprising the following method steps: generating a change in the flow direction between a transverse flow of an electrolytic fluid through the semipermeable membrane of the filter and a longitudinal flow of the electrolytic fluid along the semipermeable membrane of the filter or a change between a longitudinal flow of an electrolytic fluid along the semipermeable membrane of the filter and a transverse flow of the electrolytic fluid through the semipermeable membrane of the filter, measuring the conductivity of the electrolytic fluid flowing in the transverse direction downstream of the filter and/or of the electrolytic fluid flowing in the longitudinal direction downstream of the filter, monitoring the change in the conductivity after the change in the flow direction of the electrolytic fluid from a base value and concluding that there is an incorrectly functioning state of the filter on the basis of the change in the conductivity after the change in the flow direction of the electrolytic fluid. generating a change in the flow direction between a transverse flow of an electrolytic fluid through the semipermeable membrane of the filter and a longitudinal flow of the electrolytic fluid along the semipermeable membrane of the filter or a change between a longitudinal flow of an electrolytic fluid along the semipermeable membrane of the filter and a transverse flow of the electrolytic fluid through the semipermeable membrane of the filter, measuring the conductivity of the electrolytic fluid flowing in the transverse direction downstream of the filter and/or of the electrolytic fluid flowing in the longitudinal direction downstream of the filter, monitoring the change in the conductivity after the change in the flow direction of the electrolytic fluid from a base value and concluding that there is an incorrectly functioning state of the filter on the basis of the change in the conductivity after the change in the flow direction of the electrolytic fluid.