A method for verifying the reliability of ascertained measurement data of an ultrasonic, flow measurement made according to the travel-time difference method, wherein an ultrasonic flow measuring device having at least two ultrasonic transducers is used to transmit and receive ultrasonic signals inclined in or counter to a flow direction of a measured medium. A first ultrasound disturbance signal is registered within a first time window before receiving a first ultrasound wanted signal, and a second ultrasound disturbance signal is registered within a second time window before receiving a second ultrasound wanted signal. A quality criterion is ascertained for evaluating the measurement uncertainty of a measured value proportional to the travel-time difference ascertained from the first and second ultrasound wanted signals, wherein the ascertaining of the quality criterion includes a difference forming between the first and second disturbance signals.