Nowadays refractive-index engineering has become a challenging area for experimentalists in semiconductor integrated optics, whereas design constraints are often more strict than both standard technology tolerances and model accuracies. In fact, it is crucial to non-destructively evaluate thicknesses and refractive indices of a multilayer waveguide independently, and to this aim we resorted to X-ray reflectometry and effective index measurements on MBE-grown AlGaAs waveguides, respectively. With the first technique interference effects (Kiessig fringes) arise, which are related to layer thicknesses. By standard data processing, thickness accuracies of +/- 0.05 nm are readily achieved. Effective index measurements were performed at several wavelengths on both slab and rib waveguides, through grating-assisted distributed coupling with both photoresist and etched gratings. Effective indices were determined with an absolute precision as good as 1/2000, adequate for phase matching in parametric devices. Merging thickness and effective index evaluations, the refractive indices of the constituent layers were determined with unprecedented accuracies, in substantial agreement with existing models.