Hi Joćo,
A measure that would give something near 1.0 for white noise and near 0 for a sine wave would be "spectral flatness", which is in the timbreID library. But if you're looking to see how well a spectrum's partials line up harmonically, you won't find that in timbreID yet. One quick option would be to use sigmund~ to get the current pitch, then search the spectrum for the amount of energy in bin ranges related to the expected set of harmonics. Compare that with energy in non-harmonic bins. But then, for things like gongs that sound "pitchy" but have inharmonic spectra, that won't be much help. Depends a lot on what you're trying to do.
You *might* find specSpread~ useful, which measures how widely or tightly energy is concentrated around the spectrum's center of gravity. It's in units of Hz though.