Assuming these are audio devices?, the resonance will either be fairly low amplitude, or outside it's normal operating frequencies. It's it's a piezo beeper, these are sharply resonant, and should be operated at that frequency.
As for ultrasonic transducers, stick the scope across one of them, and feed it from the signal generator via a resistor - this way you can test each one individually. Notice that the transmitter is series resonant (minimum amplitude at resonance), and the receiver is parallel resonant (maximum amplitude at resonance).