A system and method detect at least one mechanical locking position of a drill string in a soil drilling machine. According the system and method, there are: a sensor apparatus which is configured to detect first data indicating the position assumed by the top of the drill string and second data indicating the position assumed by the bottom of the drill string. A control unit calculates the estimated length of the drill string based on the data and to compare the estimated length with a reference length value. A pair of kelly rods is in a mechanical locking position; and a signaling apparatus is configured to signal when the estimated length corresponds to a reference length value.