I know a guy who likes to talk about himself. When you see him, he'll tell you what he's been up to, how his workouts are going, and what his plans are. If you start to talk about yourself, he'll mumble "I gotta go now," and leave.
If I were the only one who'd noted this pattern, I might assume it was due to me being so boring. But I've heard others say they've noticed the same thing. He's a pleasant guy, I've never seen him get nasty, and he can be generous. But he only wants to talk about himself.
When my children were on the local swim team, I would often chat with the other parents. The majority of the parents were polite, and would ask how my children were swimming. But there was a subset of parents, a small but unmistakable minority, who would come rushing up to me, tell me what amazing time their child had just done, wait for my approval (which I would usually give), and then go flouncing off. Again, these were not nasty people; but their subject of conversation rarely varied.
Occasionally their children would actually have done a noteworthy time. But it was always the same parents who would talk only about their children, while never asking about anyone else's. And the polite parents would never fail to ask about other people's children. Eventually, both sets of behaviors became very predictable.
My only regret was that I gave the first set of parents my approval.
How much time a person spends speaking about himself, vs. how much time he is willing to spend talking about you (or at least a neutral subject), is a very direct measure of his narcissism.
Or at least of his stupidity and self-indulgence.
Think of everyone you know in those terms. It's a worthwhile exercise.