Strong Willed or Narcissitic?

My sister posed a question on her blog that I am exploring myself.

http://theprojectbyjudy.wordpress.com/2013/06/07/narcissistic-versus-strong-willed/

How do I tell the difference between someone who is self-confident and flawed and someone who is truly narcissistic?

A comment from Roots to Blossom is a great answer that I think would be helpful to see as a post:
I think the difference here is that all people, being human, have some narcissistic qualities, but only a few have actual pathologies where the narcissism rules over every part of them and others in their path.

Pathological Ns devour people, they leave you feeling dirty, guilty, used, worthless and empty. I have some self-absorbed friends I love dearly, and although I know we’ll mostly talk about them, I don’t feel used up by it. She also repays the compliments and truly appreciates the attention – and here is another difference – she is satisfied with the attention and appreciative of it, not full of ridicule and lament for my attention not being ever enough.

To put it bluntly, for me, contact or conversation with an N is like mental rape. It leaves you full of self-doubt and takes time to recover your balance.

I know many people casually toss out terms like my mom is such a narc or psycho. I just nod, knowing they really don’t understand the depth of those terms and are describing a trait, not a monster. Again our pasts just make us experts, whether we want this knowledge or not. So it is up to us to understand the point of view of those that don’t know true monsters.

And I think I hear your fear that you are also an N at times. Ns do not recognize hurtful behaviour or look inwards like this, so I can assure you that you are not. Stop hating your own emotions and reactions that make you human. I’m trying to do the same. We’re allowed to experience the entire range of emotions, even the negative ones. That makes us authentic – also something else an N will never be.

I think this will help me in the future with encounters with people that may seem like narcs and aren't and those that I am not sure why but I come away feeling used up.