Since I don’t know anyone on a personal level who shows at the levels your cite --my answer is based on 58 years of knowing other horse people in various situations, and myself! I evaluate a good horse-human relationship on lack of complaint by both horse and human.
A human in a good relationship with his horse (child, spouse, partner) doesn’t complain about the other --in fact, he/she may slip in the occasional, “I enjoy riding (having, being married to, seeing) --add name of entity.” There are the non-verbal tells --the quiet touch, the kiss on the nose, the extra brushing after working out --the extra effort to make sure the equipment involved is clean and well-fitted.
The horse (child, spouse, partner) is equally accepting and warm toward the human --there is a relaxed-ness, a trust, a calmness when the special human is around. The horse with the good human relationship may not display overt affection (they aren’t dogs! so no tail wagging), but seeks contact --touching, leaning in, watching the human, willing to do what is asked —ok, my Hancock gelding doesn’t do that, he is too dignified to acknowledge humans exist --but he does seem relaxed around me and seems to take moderate joy in the riding we do, and sometimes falls asleep when I brush him in crossties . . .
So my short answer? A good horse/human relationship is manifested by the comfort each has in the other.