Obtain a voltage versus time plot of an electronic waveform for the electronic component you want to know about. You can ask the component's manufacturer for a data sheet that illustrates the slew rate.
Note the region on the waveform plot where the voltage rises from a minimum voltage level to a maximum voltage level. Write down the highest voltage and the lowest voltage. Look on the vertical axis of the plot to find the corresponding voltage levels. Also write down the time the lowest voltage occurs and the time the highest voltage occurs. Read the times from the horizontal axis on the plot. Use, for this example, a low voltage of 0 Volts occurring at 1 second and a high voltage of 5 Volts occurring at 4 seconds.
Calculate the voltage change. Subtract the lowest voltage from the highest votage. Conclude that for a high voltage of 5 volts and a low voltage of 0 Volts, the voltage change is 5 Volts, since 5 minus 0 is 5.
Calculate the time change. Subtract the time at which the lowest value voltage occurs from the time the highest value voltage occurs. Conclude that if a low voltage of 0 volts occurred at time, 1 second and that a high voltage of 5 volts occurred at time, 4 seconds, the time change would be 3 seconds, since 4 minus 1 is 3.
Calculate the slew rate. Divide the voltage change by the time change. Conclude that if the voltage change was 5 volts and the time change was 3 seconds, the slew rate would be 1.66 Volts per second, since 5 divided by 3 is 1.66.