difference between a normal person with narcisstic traits and a NPD person?

Ever since I have joined the forum and have been reading about the NPD, I have started to notice narcisstic traits in so many persons, some of whom i previously would not see anything wrong with them.

While many therapists have admitted that even normal persons may be having some traits of some personality disorders, how are we supposed to make the difference between a full blown NPD person and a normal person with narcisstic traits?

I have no doubt that my XN has some typical narcistic traits: the ST, the entitlement, the grandiosity, the lack of empathy, etc... the list goes on. but i have no idea if he is indeed NPD or just some traits.