it is a very broad question without specific simulation/device.
The simplest way is to excite left/right circular polarizarion and quantify their results.
If you want to quantify the polarization from the output fields, you can decompose them, for exmple
E1=sqrt(2)/s*(Ex+jEy)
E2=sqrt(2)/2*(Ex-iEy)
As for which one is left or right circu polarization it is up to you from your definition since there are two opposite definitions.
if you can decompose H fields, you will be able to get the correct transmission by following  https://optics.ansys.com/hc/en-us/articles/360034405354-transmission-Script-command
Following the same procedure, you can decompoae the fields anywhere inside the simulation volume if you want.
This is a more theoretical question than simulation.
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