Measure DC resistance. If it's infinite, it's probably piezo. If it's 50-1000 ohms, it's probably dynamic.

Place the two units facing one another about 2 cm apart.

Attach oscilloscope to receiver, sensitivity 50 millivolts/div or better.

Apply signal generator to Transmitter. (If piezo, use about 5 volts RMS. If dynamic, use about 10 milliwatts).

Slowly sweep the frequency from about 25 KHz to 100 KHz. At one or more frequencies you may see a signal on the receiver.

Disclaimer: This may damage your transducers, because it is an experiment performed with no knowledge of the part.