A testing device includes a first member to be positioned on one surface of a part to be tested. A second member to be positioned on an opposed surface of the part to be tested. One of the first and second members have an ultrasonic transmitter and the other has an ultrasonic receiver. At least one of the first and second members have at least one magnetic element and the other of the first and second member have at least one magnetic or at least one ferromagnetic metallic element such that when one of the first and second members moves along the surface of the tested part the other of the first and second members will move along an opposed surface with the one of the members.