The narcissist is a man who is in love with himself. It’s easy to spot the obvious narcissist (like a sociopath) but it can be more difficult to the more subtle narcissist, as it can masquerade an overly-healthy sense of self. There is also the problem of not wanting to unfairly label someone, so we sometimes hide from the truth.
So what should you look for? The classic sign is an inflated sense of self. Arrogance and entitlement are fairly apparent in one’s personality. It can be harder to discern a lack of empathy (watch for how they react to the pain/misfortune of others) or a fragile self-esteem (do they over-react to criticism).
A lot of successful men are somewhat narcissistic, (which is why so many of them are surprised when they’re caught doing foolish things, they thought they were too smart to get caught). Narcissists can be quite charming and their belief in themselves can be quite seductive. If you get ensnared in their web, it can take a while to see the truth.
So perhaps being involved with a narcissist is not so black and white. While the easy advice would be that you should run immediately, perhaps the better advice is to tell you to determine how much of a narcissist he is and to ask yourself if can you live with that? Is the upside worth the downside?