We present an experimental method allowing to quantify slip at the wall in viscous polymer fluids, based on the observation of the evolution under simple shear flow of a photobleached pattern within a fluorescent labeled polymer melt. This straightforward method provides access to slip length at top and bottom interfaces in the 1 mu m to 1 mm range and to the actual shear rate experienced by the fluid. Based on simple optical imaging and image analysis techniques, this method affords an improvement compared to previously reported methods in which the photobleached fluorescence intensity profiles before and after shear were compared and measured by scanning a photomultiplier. The present method relies on a direct determination of the displacement profile inside the polymer fluid from an analysis of the 3D evolution of the whole photobleached pattern. We demonstrate the potential of this method with measurements of the slip length for an entangled polydimethylsiloxane (PDMS) melt, as a function of the shear rate, in contact with several weakly adsorbing surfaces, i.e., end-tethered PDMS or polystyrene (PS) chains, a self-assembled monolayer (SAM) of trimethoxy(octadecyl)silane (OTS), and a glassy PS thin film.