A tool for fixing a tension clamp including a pair of levers pivotable about parallel axes and carrying at their ends plier jaws that can be moved toward each other and act on the clamp. To monitor the forces occurring during the locking of a buckle and a clamping band of the tension clamp, use is made of a device that has a housing fixed with respect to the tool and power transmission arms. Each power transmission arm is at one end elastically articulated to the housing, has at the other end a contact surface for a force sensor (18) attached to the housing and has, between its ends, a bore, in which the axis of the relevant tool lever is supported. A force/time or force/path curve obtained from the sensor signals is used as a measure of the quality of the locking between the buckle and the clamping band.