Very interesting question!!
As a rider, I know I work well with my trainer when I understand everything he tells me and when I really feel that everything has sense and my horse is happy working together! My horse tells me we are in the right way!!! :-)
As a trainer, I know when my students are working well with me when I see their face!! Thery are happy! They enjoy! They improve!!
I started having fun while taking lessons and working with her and she helped bring out my horse's full potential!