How, when do you know?

Forget cultural differences for a moment. How do you really know a person? When do you know? Is it a time frame or a series of hurdles (natural or constructed) or simple repetition or something else entirely? How often do people let you into their motivations, and, even if you know these motivations, how do you know you won't be sacrificed for them? When does trust enter? When does it become complete? I've achieved trust - in the purest, most sincere meaning of the word - with several, many even. But, as I am isolated in an environment that is foreign and lacking all the comfortable distractions that typically make the drive for faith less obvious, I want to know when and how this happens. Can I make it happen? Do I explain and reexplain what I want, what I need, to someone and hope they "get it"? Or should they just be expected to get it on their own? If you explain in graphic detail what it is that serves as your center, your heart, your mind, how do you know if someone follows directions well or if they truly understand? Does it matter? Should it matter, I guess. The difference matters to me - my life isn't a workplace where I just want people to fulfill some basic job duties correctly - the ends and the means matter. When incomplete, what breaks trust? A simple slip shouldn't do it, but what about repeated slips - off-hand comments, lack of courtesy, a tendency to value the new above the old? Where is the line between human error (deserving compassion and forgiveness) and showing one's true hand? Where's the point between being a cold, skeptical hardass and a blind, manipulated chump?