1. A method for inferring, using a control unit including a processor and a memory, a faulty or properly functioning exhaust-gas sensor in an exhaust duct of an internal combustion engine, the method comprising: operating, via the control unit, the internal combustion engine in a first operating point, wherein the internal combustion engine is operated in a trailing-throttle operation, and wherein the trailing throttle operation is not provided during regular operation of the internal combustion engine and is requested separately to implement a trailing-throttle adaptation of the exhaust gas sensor, which is a wideband lambda oxygen sensor; performing a first function control of the wideband lambda oxygen sensor during the trailing-throttle operation which is suitable for functionally controlling the wideband lambda oxygen sensor; monitoring, via the control unit, the exhaust-gas sensor in the first operating point; determining, via the control unit in the first function control, whether the exhaust-gas sensor is operating properly in the first operating point; if it is determined that the exhaust gas sensor is operating properly, operating, via the control unit, the internal combustion engine under partial load in at least one second operating point, wherein the second operating point is activated in a selective manner or is detected during regular operation of the internal combustion engine; monitoring, via the control unit, the exhaust-gas sensor in the at least one second operating point, if the exhaust-gas sensor is operating properly; determining and storing, via the control unit, as a learned value, an output signal of the exhaust-gas sensor or a characteristic quantity derived therefrom in the at least one second operating point; assigning, via the control unit, an upper and a lower threshold value to the learned value; comparing, via the control unit, the output signal of the exhaust-gas sensor or the characteristic quantity derived therefrom to the learned value during a subsequent operation of the internal combustion engine in the second operating point; and inferring, via the control unit, a faulty exhaust-gas sensor if the output signal of the exhaust-gas sensor or the characteristic quantity derived therefrom exceeds the upper threshold value or drops below the lower threshold value.