It may be bit misleading to think of it as a continuum with a line. It seems anything except exactly healthy self-confidence is a brand of disassociation from others, and might be called narcissism, either over- or under-valuation of self. I like the saying that "modesty is the highest form of arrogance". Since healthy confidence places the self on a level of equality with others, then an unhealthy state would be based off comparisons with others, and so sees the self as existing in a social power hierarchy. But if it is a sense of power which dictates ones comparisons, then there is no "just right" level. The whole conception of the hierarchy is flawed, and fundamentally cannot produce the right answer from within it. That is why thinking of it as a continuum would be misleading.
The whole concept of power dynamics makes visible the dissociative element of the disorder, and you notice that the power hierarchy must be based on competition, or else comparisons could not have meaning. But if it is based on competition, then shame or guilt is implied in falling to the bottom. Otherwise, it would just be for fun. If it were just for fun, then there would be no power-based struggle. Since it is not just for fun, then fear is involved in the motivation for power. So, our breaker of social ties is fundamentally afraid.
Since fear is always of something (or else it could produce no motivation away from it), then you can tell the difference between health and lack of health by asking yourself "what is at stake?" If something is just for fun, nothing is at stake. Otherwise, something is.
Remember that the mind will often say nothing is at stake when something is, so you you must be vigilant and watch the mind's activity and be honest with yourself about what you see. Then, you will always know, when dealing with others, whether your approach is healthy or not. Fear is not difficult to recognize because it will always fill you with a sense that something is missing or out of place, so be honest about your feelings.
"Narcissism" gets a very bad rap, but I think you can recognize your distance from it by the degree to which you are able to look upon it with forgiving eyes, without mocking it or hating it, and so on.