That's not the real problem. We deal with this problem every time in our legal system and we've developed the so called "prima facie" evidence or rulings, that seek to minimize immediate harm.

If a policeman sees a guy wielding an axe chasing someone and shouting "Come here you mother fucker!", the policeman can't just go on his way saying <<Well, a crime was not yet committed and "Come Here" is not really a threat>>. There is prima facie evidence of an aggression and he has immediate authority to arrest the assailant for a limited time, and then justice can run its course.