By analyzing each and every one of them with scripts simulating mouse movement
Some of the scope FOV's are linked to their power, but they can be based on HFOV, VFOV or perceived zoom, while others have absolutely no relation to their zoom power name at all. Data mining done by others can help support the analysis, but due to the wast amount of different engines and protected source codes this is not a method to rely on.
So to take out any guesswork they are each analyzed individually.