Widening of pupils
	Clenching of fists
	Shifting into a fighting stance.
	Looking or slowly turning away (setup for sucker punch)
	Shoulder twitching
	Swaying back and forth
	Arm cocking for punch
	Knee lifting for a kick
	Jaw clenching

Upon noticing these physical signs one should either retreat into a guard position or strike immediately. In Karate there is no first strike, I believe if someone shows you aggressiveness with the above physical signs that individual has the intent to hurt which is the first strike.