We present a new method to measure the viscosity of a dense model suspension using an inclined plane rheometer. The suspension is made of mono-disperse, spherical, non-Brownian polystyrene beads immersed in a density matched silicon oil. We show that with this simple set-up, the viscosity can be directly measured up to volume fractions of phi=61% and that particle migration can be neglected. The results are in excellent agreement with local viscosity measurements obtained by magnetic resonance imaging techniques by Ovarlez [J. Rheol. 50(3), 259-292 (2006)]. In the high density regime, we show that the viscosity is within the tested range of parameters, independent of the shear rate and the confinement pressure. Finally, we discuss deviations from the viscous behavior of the suspensions.