To measure the frequency response of any amplifier or network, you need a calibrated signal generator, AC RMS meter and some graph paper. Connect the meter across the load, and set the generator to a low frequency, say 20Hz at 50mV.
Record the reading on the meter. Next increase the signal generators frequency output to 40Hz but maintain the same level (50 mV ). Record the reading on the AC voltmeter. Repeat these steps in even increments and record the meter reading for each frequency that you measure.
You will find that at both high and low frequencies, the meter reading will start to fall. At other frequencies the output maybe be uniform ( an amplifier ) or have sharp peaks or troughs ( a filter or frequency selective amplifier ).
The results can be plotted on graph paper, horizontal ( x-axis ) for the frequency and vertical ( y-axis ) for the gain.The whole process is time consuming, but can be speeded up by using a logarithmic scale for measurement.