We introduce and experimentally apply "cosine similarity" as an index for quantitatively evaluating the degree of change in the spectra of optical frequency combs. The cosine similarity with the original spectrum increased or decreased as the amount of control applied to the combs increased or decreased; this is considered to be an appropriate indication of spectral similarity. Therefore, we apply this approach to an evaluation of the temporal spectral changes in polarization-maintaining (PM) and non-PM combs. The results suggest that there is no significant difference between the spectral stabilities of PM and non-PM combs, and reveal that the spectral sensitivity to the amount of control is a more effective factor.