Any antenna is designed to radiate or receive certain polarization. The design results also in ortho- to copol ratio. For good results, this ratio should be more than 20 dB but it also depends on the system in which antenna will be used, and the opposite antenna (sometimes the same type, sometimes not).
If your antenna is linearly polarized, the 3D pattern indicates a maximum and a minimum, so you can estimate the ortho-copol ratio from it. With circular polarization you must get the 3D patterns fo each polarization for it.
Real antennas can be measured against a calibrated reference antenna, best on an open range.