We present a phase-shifting holography method in k space that allows us to investigate the phase and amplitude of optical Bloch waves. The method is based on a Fourier space imaging technique combined with a Mach-Zehnder interferometer. Using a W3 photonic crystal waveguide as a model system, we determine, with high accuracy, both the intensity and the phase of the envelope function of the Bloch modes propagating in such waveguides. We show that local variation of the dielectric map as low as 20 nm can be clearly observed from the part of the angular spectrum of field located inside the light cone.