We evaluated the accuracy of the standard m-line method in the description of the refractive index profile of ion-exchanged waveguides by comparing its results with those from energy dispersion spectroscopy and direct near-surface/extended Brewster–Pfund (DNS-B) Method. Looking at a few single and double-exchange processes, the comparison shows three major situations: (i) the m-line accuracy is validated by the other two methods; (ii) discontinuity of ion-concentration does not allow the assumption of a continuous refractive index profile; or (iii) although the overall m-line profile is compatible with results from the other two methods, these significantly depart by a near-surface inflection-in which case, the DNS-B can be used together with the m-line experimental data to produce a more accurate index profile.