Detecting the mobility of a fill level echo according to the following steps. The echo characteristic of a first echo of the echo curve and an echo characteristic of a second echo of the echo curve are determined. This is followed by calculation of a mobility value of the second echo with the use of the first echo characteristic. In this way a situation can be achieved in which the differences in the mobility values of echoes relating to the fill level and those echoes that are due to container internals become greater.