Narcissistic Personality Disorder (or NPD) falls on a spectrum. It can look different in different people and various situations. This spectrum of presentation is one of the reasons it can be incredibly challenging to know if you're in a relationship with a narcissist.
Being in a relationship with a narcissist can make you second guess everything you know, making you constantly question yourself. This uncertainty makes it even more challenging to tell if your partner is a narcissist.
Narcissists often cut you off from your support system of family and friends. They lie to make you sound crazy. They manipulate your loved ones into not believing you. Frequently this means, you don't have anyone to turn to for help, no one to validate what you're going through.
The DSM (Diagnostic and statistical manual of mental disorders) lists nine criteria for NPD, but someone only needs to meet five of them to qualify as a narcissist clinically.