what are the subtle signs of narcissism in relationship which one needs to lookout for

I've known people who'd crib about being alone but anytime they're approached by someone who'd actually care for them, they'd treat them like shit. In reality they don't seem to like anyone enough to 'bear them'and it's not just they're picky, they get a sense of superiority by rejecting potential people. I used to think that people who would constantly talk about themselves or who had a larger than life image of themselves were narcissistic but then i met someone who used to make self depreciating remarks all the time... That made them seem like the opposite of a narcissist, but little did i know that while doing so, they still were the centre of their own universe. I wish we had more awareness of such red flags in relationships.