We report on a room temperature detector which allows to determine and monitor the state of polarization of terahertz radiation with picosecond temporal resolution. The detector is based on the circular photogalvanic effect recently observed in GaAs/AlGaAs quantum wells. The circular photogalvanic effect yields in response to elliptically polarized radiation a current signal proportional to the degree of circular polarization. The peak current signal occurs in unbiased samples for circular polarization, vanishes at linear polarization and changes sign by switching the helicity from right-handed to left-handed. The detector consists of a (113)A MBE grown p-GaAs/AlGaAs multiple quantum well structure. The response has been measured in the wavelength range between 76 mum and 280 mum at normal incidence of the radiation on the sample.