  The characterisation of loss in optical waveguides is essential in
understanding the performance of these devices and their limitations. Whilst
interferometric-based methods generally provide the best results for low-loss
waveguides, they are almost exclusively used to provide characterization in
cases where the waveguide is spatially single-mode. Here, we introduce a
Fabry-P\'erot-based scheme to estimate the losses of a nonlinear (birefringent
or quasi-phase matched) waveguide at a wavelength where it is multi-mode. The
method involves measuring the generated second harmonic power as the pump
wavelength is scanned over the phasematching region. Furthermore, it is shown
that this method allows one to infer the losses of different second harmonic
spatial modes by scanning the pump field over the separated phase matching
spectra. By fitting the measured phasematching spectra from a titanium
indiffused lithium niobate waveguide sample to the model presented in this
paper, it is shown that one can estimate the second harmonic losses of a single
spatial-mode, at wavelengths where the waveguides are spatially multi-mode.
