The catch is that the bullied is often not well positioned to tell reliably whether bullying happens or not. We can assume it's friendly banter while it's bullying. As a child the idea that someone might want to hurt me just did not compute and caused a paralysing response. Or we can assume it's bullying even though it's friendly banter. After being bullied I saw any tease or taunt as a personal attack and this cost me close friendships in uni years.

Here's a test; if you express discomfort in a defensive way and they double down, it's probably bullying. If they back off, it was probably good natured. If someone interprets your discomfort as vulnerability they can exploit - there you go. (It's probably not worth conducting this test, josephg's advice to get sincere instead of defensive elsewhere in the thread is probably the best response in either case.)

Not foolproof but I think it's a solid rule of thumb. I dunno if I just have a different set of social gifts and deficits from other people in this thread, but I don't feel like I have much trouble determining when someone gleefully manipulates my emotions to wind me up. People who are genuinely just teasing don't get excited if they upset you.

I think the difference in thinking is that when people think of bullies, they seem to be thinking of being a child or young adult. But I've been treated in ways I regard as bullying at every point in my life. I'm sure that I was just as prone to misinterpretation as a child as anyone else.

So I'm thinking of applying the emotional sophistication of an adult to the situation, and it seems clear to me that this is something you can figure out, because I do it once or twice a year. I'm guessing other people do too, but they probably think of those people in different terms, like "asshole."

Yes, that's what I did, express discomfort and they backed down. But that put me basically outside of those circles, since close friends would tease each other as a sign of familiarity. If you indicate you don't like that and want people to be polite with you, it creates distance. That's basically what politeness is, walls and emotional distance.