The Latin root for the word “assessment” is assidere, which means "to sit beside".
Teachers/parents/mentors know this and understand it in a profound (even if only intuitive) way. They understand that to be able to truly assess the progress of a child/student/mentee implies a deep understanding of the other, and the contexts that have a bearing on the other's learning. Inherent in this relationship is trust and mutual respect.
The wisest teachers/parents/mentors know that authentic assessment is not and cannot ever be accomplished via a standardized test -- it's more of an ongoing conversation between the guide and the learner.
Superficial attempts at assessment produce superficial assessment results, which are at best bogus and hollow.
Assessment of learning is, and always will be, a messy and ambiguous process. And, it is and always will be best administered by caring, wise, fully-invested teachers/parents/mentors.