We compare three optical models of graphene, namely, the interface model, the isotropic model, and the anisotropic model, and verify them experimentally with two multimode slab waveguide samples operating at the wavelengths of 632.8 and 1536 nm. By comparing the calculated graphene-induced losses and the measurement data, we confirm that the interface model and the anisotropic model give correct results for both the transverse electric (TE) and transverse magnetic modes, while the isotropic model gives correct results only for the TE modes. With the experimental data, we also quantitatively verify the widely used expression for the surface conductivity of graphene in the optical regime. Our findings clarify the issue of modeling graphene in the analysis of graphene-incorporated waveguides and offer deeper insight into the optical properties of graphene for waveguide applications.