Determine if Fn + Key can be made to produce keycode if xev provides none

In relation to the comments on this answer, I am curious about the following:

If one does not get a keycode output in the xev Event Tester when pressing Fn + Key, are there ways to determine whether obtaining a keycode for the combination is possible?

As far as I have understood, some keyboards have the possible options hardcoded, while others might be softcoded. I would like to know if there is some way to determine if one or the other is the case, without decontructing the physical keyboard.