Her ex is a narcissist, but what about her?

Have you ever noticed that as a single person, it seems so many people are experts at identifying their exes as Borderline or Narcissistic Personality Disorder?

It's common to hear these terms thrown around. It either makes you sympathetic for the person and what they've gone through (especially if they're attractive) or concerned that they're throwing off their responsibility for their issues by blaming their ex. 

Additionally, when everybody seems to be Borderline, Narcissistic, drug and alcohol addicted, unfaithful, liars, or manipulative, how do you know that you're not dating somebody who is also that way?

It doesn't seem safe, as if the people you're dating can change on a dime.