A method and a control assembly for operating an exhaust gas system of a motor vehicle is disclosed. Measuring values are evaluated, which indicate a content of nitrogen oxides in an exhaust gas downstream of a catalytic device. The catalytic device is adapted to diminish the content of nitrogen oxides in the exhaust gas produced by an engine of the motor vehicle. Based on the measuring values a quality of a reducing agent supplied to the catalytic device is assessed. The method includes determining whether reducing agent has been filled into a storage tank. A plurality of measuring values is captured during a predetermined period of time, and a magnitude and a frequency of the plurality of measuring values are taken into account to assess the quality of the reducing agent.