Abstract: A testing device for tying a yarn loop on an agricultural bale forming machine having a bale forming chamber, has a sensor finger extendable transversely into a first passageway of an initial portion of a loop before a knotting step and also into a second passageway of a finished loop after the knotting step and holding the initial portion of the loop and the loop differently in different directions, an electrical switch movable to a switching-over position when the sensor finger is actuated so as to deviate by the loop because of the bale ejecting movement, the sensor finger having an end region formed as a spring which holds a passage of the initial portion of the loop deviatingly and during deviation of the electrical contact is at least indirectly switchingly actuated by the loop.