The line between harmless fun and bullying
If we find ourselves asking what the line is between harmless fun and bullying with regard to our own behavior, then rest assured, there is a good chance you have already crossed that line. Nothing is absolute, but having fun at another's expense is always unecessary and a wasteful use of time in this short life.
Perhaps you are simply attempting to assess previous behavior where you were hanging out with a friend or friends and one person in particular got into a nudging match with you. Neither of you may have had harmful intentions, but you were both simply enjoying using your brains in that unusually used capacity. This sort of scenario really does not qualify in this topic.
If you are wondering about the line between harmless fun and bullying for yourself, a helpful suggestion would be to simply not even go anywhere near any kind of behavior that may be construed as bullying.
Bullying is attempting to force your will on another. This may be a physical, mental or emotional sort of bullying, but whatever way you slice it, it is about forcing your will. Did the harmless fun accomplish anything that could be categorized under attempting to force your will on another person? If so, then you have crossed the line and you ought to retreat back across to the other side, perhaps even apologizing on your way back.
If you are wondering about this line because of the way that someone else treated you, then just keep it simple and ask yourself if they made you feel badly and if you felt forced in any way whatsoever.
Perhaps this line question has come up simply in the thought of learning about and learning from another person's life and their behaviors. Maybe someone told you something negative about someone else and you are trying to assess whether your opinion of them should change or whether or not you'd like to investigate the matter further.
Whatever the reason may be, always go back to that question of whether or not another's will was forced. If no, then it is not bullying. If yes, then it's bullying all the way!
What kind of harmless fun might you be thinking of at this point? Is it possible that it truly is just harmless fun and for some weird reason you may have in your mind the thought that at this point in your life you need to be constantly aware of whether or not you are bullying anyone else. This could have come on through self-examination or perhaps through the advice of others. Whatever the cause of that is though, it's a good thing to recognize all that what bullying truly is and that as long as we keep in mind the need to not cause harm to others, neither mentally or physically, then this particular question which may have been knawing at your psyche, may now be good as gone!