We present an experimental method for direct analysis of guided-mode resonances in photonic crystal slab structures using transmission measurements. By positioning the photonic crystal slab between orthogonally oriented polarization filters light transmission is suppressed except for the guided-mode resonances. Angle resolved transmission measurements with crossed polarizers are performed to obtain the band structure around the Gamma-point. Results are compared to mode simulations. Spatially resolved measurements in a confocal microscope setup are used for homogeneity characterizations. Stitching errors and inhomogeneities in exposure dose down to 1.3% in photonic crystal slabs fabricated by electron beam lithography are observed using this method.