We retrieve the dispersion properties of photonic crystal waveguides near the band edge with high experimental accuracy. The dispersion diagram of the waveguide modes in the complex-valued plane is directly measured in the far field by using a Fourier space imaging technique. We show that the investigation of the modes in k space provides a clear signature of the transition between propagating, evanescent, and localized modes. It allows us to determine the impact of the structural disorder and of the dissipation on the group velocity of the propagating wave in the slow light regime.