Below I demonstrate how it works for built-in RCWA dll but it could expand to other diffraction mode. The trick was shared by Yihua using NSRA with only one order observed in one configuration.

For built-in RCWA surface, the trick is to use the parameter "only these orders", this is explained in detail in the KBA. Simulating diffraction efficiency of surface-relief grating using the RCWA method – Knowledgebase (zemax.com)

We can check if the parameter has been set correctly in the 3D layout to see if there is only the order that we want to observe.
Then we are interested in the last segment of the ray, so we use the NSRA to see the direction cosine of the last segment. 
The direction cosine can be read by NSRA 4-6.