In order to perform a cleaning-in-place or CIP cleaning of a filling element of a filling machine for the filling of containers with a liquid filling content, at least one closure element can be used, together with the filling element, to form a flushing chamber, in which a filling valve and a filling-level probe are at least partially disposed for the cleaning-in-place cleaning of the filling element. A control unit controls the flow of cleaning medium into the flushing chamber and the performance of at least one electrical measurement with the filling-level probe during the cleaning-in-place cleaning of the filling element.