With an audio editing software that has harmonic analysis capability, as you turn the knob, you can easily follow the resonant frequency on the frequency analysis graph since it's the peak.
You could also do it by ear, comparing the tone with another (tuned) source, like the 2nd oscillator. The frequency is then obtained by a table that gives for any tone the corresponding frequency.
For every key position of the knob, you take note of the peak frequency, which happens to be the cutoff frequency and soon enough, you have calibrated the cutoff frequency on the Korg DS-10.
There you go, the cutoff frequency knob and the (approximate) cutoff frequency values.
On a practical note, say you have a sound patch (from a book) and you know the cutoff frequency to apply but don't know the corresponding knob position. What you can do is play the corresponding pitch on one oscillator and tune the cutoff frequency (at maximum peak) on the other oscillator to that pitch. With this method, there's no guess work involved (beside the pitch matching).
Next in the program is the calibration of the envelope generator: attack, decay, sustain and release.