If you want some degree of accuracy in your measurement, you should use an oscilloscope.
You would be able to actually see the collector to emitter voltage waveform of the output transistor.
A scope image will allow you to establish an average value for the transistor voltage while it is turned on, and to verity the duty cycle and period.
To measure the current, you could insert a low value resistor or shunt in the output current path and get a reasonably accurate idea of the actual current waveform.
These two values, voltage and current, will provide sufficient data to find the output power.
It does not seem likely that you will obtain accurate measurements by using only a voltmeter or an ammeter.