Quick and dirty method... Just measure the self resonance of your coil. If you calibrate the nVNA to your fixture and then just run the S11 sweep up high enough on the coil, you will undoubtedly see a resonant peak. At low frequency you can determine the inductor, L. (the nVNA saver software will do this for you) The frequency of the peak tells you the self resonant parasitic capacitance, C. From the magnitude of the the resistance, R, measured at self resonance, and the calculated self resonant impedance, Z=sqrt(L/C), then Q will be R/Z.
This procedure also shows you explicitly when parasitic capacitance becomes important -- which it often does!
