I am still working toward this boundary. I can feel myself wanting to say that automatic "yes". I still feel badly when I do not say yes. Sometimes, I say no because I know that is the right thing from the work we do with and for one another here on the site, but I don't feel it in my heart.
But I say no anyway. And that seems to have made an amazing difference for my kids. And that feeling of having a choice about things ~ that sort of feeling that I have time to think about it as opposed to that automatic "yes" seems to be percolating into other interactions in my life, too.
I can see it is true that a person with correct boundaries would know when a relationship is unbalanced. I never even had that concept, before trying to figure out detachment and how to do that.
So maybe I will incorporate that understanding, as I continue to come into a more balanced way of seeing.