A hydraulic line (1 0.1, 1 0.2, 1 0.3) of a hydraulic system is connected with one connecting device (6.1, 6.2, 6.3) out of a plurality. A control unit (7, 12) automatically detects that connecting device (6.1, 6.2, 6.3) which is in fact connected with the hydraulic line (1 0.1, 1 0.2, 1 0.3). For doing so the control unit (7, 12) supplies every connecting device (6.1, 6.2, 6.3) of the plurality with fluid on a trial base. A detection device (15.1, 15.2, 15.3, 20) checks whether or not the hydraulic line (1 0.1, 1 0.2, 1 0.3) is in fact supplied with fluid.