The beam polarization of the proton beams is measured with a set of polarimeters at IP12.
Carbon polarimeters: fast measurement several times during a RHIC fill, two devices per beam
Hydron jet polarimeter: absolute polarization measurement with a polarized atomic hydrogen jet, for normalization of carbon devices

Detailed information on polarimetry and results are available from the RHIC spin webpage: wiki.bnl.gov/rhicspin/Polarimetry

Summary of results for the experiments: wiki.bnl.gov/rhicspin/Results