Narcissistic traits in individuals can be harmful for all parties involved, including the narcissist. And truly, covert narcissists tend to be the least likely to realize that their traits are toxic and/or maladaptive. That all being said, it's not uncommon for a person with toxic traits of narcissism to not realize they are narcissists and to accept and to be conscious of their toxic traits, let alone change them if they don't have the insight that they are there.
I have worked in a lot of capacities where narcissism is involved. This includes working with individuals that are still in a relationship with a narcissist, individuals that have left an individual with narcisissistic traits and even narcissists themselves. It some capacities, it would not have been safe to let the person with traits of NPD to know that they are a narcissist, especially if they do not see a need for treatment or change, unfortunately. In other situations, narcisisstic individuals that I have worked with have come to gain the insight into their own behaviors and have worked hard to heal those traits by looking deeply at where they come from.
Many clinicians - especially clinicians that are very data driven in their treatment methods - would say that the only true way for us to be able to diagnose a personality disorder is to use a personality inventory much like the Minnesota Multiphasic Personality Inventory (or the MMPI) and by matching up their symptoms with the criteria for the diagnosis in the 5th edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), the manual that mental health professionals use to determine what diagnosis best fits our client's needs. With that being said, a clinician would be able to most accurately diagnose someone through a series of face to face interviews with the person in question. Personally speaking, I prefer to have a treatment history of 6 months or so with an individual before making a diagnosis of a personality disorder. This is because this gives me time to see multi-faceted qualities of their personality under differing circumstances in their life.
Even if you will not be able to know for sure if your partner has NPD without getting him to commit to a diagnostic session and ongoing therapy, I wonder if you are asking this question because you are concerned about the way that your boyfriend is treating you or others in your life. If something feels toxic, it may be toxic. Often times, narcissists do things that they hope will cause insecurity in their partners to make it hard for them to know who or what the problem is. This is one way that they maintain a power differential and are enabled to continue to possess the toxic traits that they think are helping them to feel better, less out of control and secure. Feeling unsure is what the narcissist often wants and keeps you from abandoning them. You are allowed to trust yourself in knowing what is best for you. You are also deserving of existing without those doubts looming over your head. If the relationship doesn't feel like it is serving you, building you up or makes you feel confused and insecure, that is enough reason to reevaluate the relationship.