I tend to get a good sense of how attached I am in cultivating their character when I do not have to really think about how they would react; that it gets to second nature to use their thought process and personality. That, coupled with how much I spend on them to have concept art or story set pieces done is another indicator to myself.

Either way, there are some characters I have about that I never get to use but are my absolute vanguard in terms of refinement and development. Most just are not appropriate to the majority of settings and or would be disruptive, hah.