Similar to Professor Watson's suggestion for a simple visual assessment of sound output, you can take some kind of tape, preferably 1" wide or more and apply it around the US head creating a well into which you can pour water. After you turn on the US device, I suggest initially testing with a 100% duty cycle and 1 W/cm2. If the transducer head is functioning, you will see a disturbance of the water surface in what appears to be concentric rings. As you decrease the duty cycle you will see the magnitude or severity of the water disturbance decrease with the decreasing duty cycle. In this manner you can at least determine if the transducer is still providing differing duty cycles. Another neat feature of this methods of assessing an US transducer is that you can get an idea of the effective radiating area (ERA) when looking down into the water well and seeing the diameter of the area where the water ripples originate in comparison to the size or diameter of the transducer head. Not a technical or specifications check, but a quick functional assessment and wonderful teaching lesson.