Only when it damages, or bears the immediate possibility of damaging another individual. Thus libel, defamation, perjury, fraudulent statements and the infamous false cry of "fire" meet that test.
How does carrying or owning an "assault weapon" or "Saturday night special" meet this test? How are they not all protected from regulation by prior restraint?
Because the restrictions on speech are not prior restraint - they are pre-declared consequences to the outcome of the speech. Libel, defemation, perjury, fraudulent statements, etc. are what they are only because they harm, or threaten to harm, someone. Lie harmfully about someone, suffer the consequences. Lie about Bill The Cat, and nobody will care.
Mere carry of certain arms does not harm anyone. Pointing them at someone, however, is certainly cause for retribution.
Swearing in public? Speech over the airwaves? Fighting words?
Oh, and since when are our laws limited to regulating behavior that harms others? Is this some new, made-up standard of yours that we're supposed to comply with? Where did you get this standard and why are you using it?
"How does carrying or owning an "assault weapon" or "Saturday night special" meet this test?"
Test? It's considered a reasonable regulation.
"How are they not all protected from regulation by prior restraint?"
Because the legal concept of "prior restraint" doesn't apply to guns? Just a guess.