On a legal level, it's called the reasonable person standard. Nearly all US (and probably all UK common law derivative) jurisdictions use this standard in tort, contract, and criminal law. The term comes from a determination as to what the "reasonable" would do under the circumstances. In layman's terms, it's the average person. The court determines whether the person acted in a manner consistent with what the average person would have done.
In this type of situation, the RPS would essentially ask whether a reasonable (average) person would have reacted violently to those comments. If they would have, then the person saying the comments would likely be found to have incited the violence, even if they honestly did not know it would happen. Their ignorance would not be a defense. If a reasonable (average) person would not have reacted violently to these comments, then the person making them would not be found to have incited violence, even if it did occur.
Of course, this is part of state law, and thus it's impossible to state a general rule. Some states will have exceptions that say that even if a reasonable person would not have reacted violently, actual knowledge that this specific person would react violently would be enough. So, if you told someone the sky was blue and that made them punch you, you may still be liable if you knew for a fact that they would punch you if you said that.
However, all this is merely hypothetical, because the vast majority of states do not allow for escalation to physical violence from verbal exchanges, regardless of what is being said. Without knowing a specific state to reference, the odds are that no matter what is said, and no matter how malicious the intent of the words, any violent response will be a criminal act.
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