Brandenburg Test

The government can only forbid or proscribe advocacy of the use of force or of law violation if:
(1) such advocacy is directed to inciting or producing imminent lawless action; and
(2) such advocacy is likely to incite or produce such action.

Clear and Present Danger Test (from Schenck)

The question in every case is whether the words used are used in such circumstances and are of such a nature as to create a clear and present danger that they will bring about the substantive evils that Congress has a right to prevent.

As interpreted in Dennis: In each case [courts] must ask whether the gravity of the 'evil,' discounted by its improbability, justifies sucyh invasion of free speech as is necessary to avoid the danger.