Hi there, interesting topic! When I think about discernment and resonance, I guess that resonance is part of a process, an experience. While discernment is the realisation that you feel that this experience works for you or not. I don't think at all it's just mental, for me it's also intuition that is involved.
Even body language, smell and the tone of voice. And very important, being aware of my internal reactions, being a witness to that. And keeping in mind that I can make mistakes, so that I try to be careful in judging. Which is of course confronting me with my judgements.
It's part of every day life, I think, useful in every moment, I always find that this should be taught in education at home and at school.