Connect the instrument to the oscilloscope, using the BNC cable and adapter. If you're using a traditional instrument, connect a microphone to the BNC cable.
Turn the oscilloscope and instrument on.
Set the oscilloscope's horizontal sweep rate to 1 millisecond/division or faster.
If you're using a microphone, set the vertical Volts/Division to 1 or 2 millivolts. Otherwise, set it to .5 volts.
Switch the coupling to AC.
Have a friend play and hold a note on the instrument.
On the oscilloscope screen, count the horizontal divisions for two full wave cycles. Include any fraction of a screen division, since it might not be a whole number.
Calculate the frequency of the signal by dividing 2 by the number of horizontal divisions times the sweep speed (seconds per division). Example: 2 / (5.3 * .001) = 377.36 cycles per second. Refer to a conversion chart to convert frequency to musical notes (see Resources).