The process is quite simple: you route the LFO (square waveform) to the Voltage Controlled Amplifier (VCA) or simply amplitude, turn the LFO amount to the maximum setting and play/record a note on the keyboard.
Looking at the recorded signal, the time in seconds between the leading edge of one pulse and the leading edge of the next pulse is the period T of the modulated signal. Invert that time and you have the frequency f=1/T in Hertz.
Rinse and repeat for various knob positions.
LFO frequencies for various knob positions.