The physics of laser-plasma interactions has undergone dramatic improvements in recent years. By directing a multi-TW, ultrashort laser pulse onto a thin foil or a gas jet, it is nowadays possible to produce multi-MeV proton, ion and electron beams. Although much progress has been made in characterizing and improving the quality of such laser-generated beams, it is still an untouched issue whether the laser-generated beams are or can be spin polarized and, thus, whether laser-based polarized sources are conceivable. To this end, one may either think of a spatial selection of certain spin states through the huge magnetic field gradients that are inherently generated in the laser-generated plasmas, or of pre-polarized target particles which maintain their polarization during the rapid acceleration process. We have developed a method to measure the degree of polarization of protons that have been accelerated at the 300 TW laser facility ARCturus at Dusseldorf University.