In many a time we are used to a terminology but forget its original definition. The polarization state can be best defined for plane wave, but it might be difficult for non-plane waves with a few exceptions. I am not sure what the functionality of your metasurface. Is it used to create a plane wave such as a prism-like device, or it is a lens?
I am not sure how to get the polarization from s parameter since none of s is associated with electric fields. For the polarization ellipse, it is used for periodic structure so its farfield is a plane wave for any give diffraction order.
In your case to verify the reciprocity, you may simply check the near fields of the transversal E components and compare them. You will need to use phase compensation to the monitor data to get the phase change purely from the metasurface. Please refer this page for more info:
Metamaterial S parameter extraction
If you still have questions, please elaborate more about the metasurface and the wave (fields) .
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