I don't believe a standard measurement method exists. Certainly slew rate will limit transient response, but slew is a steady state measurement. Peak transient power, for example, might be an interesting thing to measure. Find a pulse generator and set duty cycle to 10% and time interval to 0.5 seconds. Measure voltage across an 8 ohm load with an oscilloscope, and turn up the amplitude until the pulse doesn't get any taller. Scope can be externally triggered by the pulse generator's trigger output to make it easier to see and measure.