There are a myriad of pre-assault or pre-contact indicators that we humans, much like animals, display unknowingly. These are involuntary physiological “tells” that project from a person when they are in fight or flight mode. All of these indicators happen as a result of the body’s response to an adrenaline dump preparing the attacker for engagement in violence. This is the most comprehensive list I have been able to aggregate, but there are surely more that I have missed or that are unique to individuals. The following video covers and illustrates a few of them- it is a bit dry but decent info.
Blading/Change of Stance: Body ‘blading’ happens when the aggressor puts his strong foot slightly behind him so that his body is bladed, setting up his strong hand to have more power (fighting stance). He is subconsciously setting up his attack.
Fist Clenching or Pumping: One of the side-effects of the fight-or-flight response is that blood is pulled from the extremities into the large muscle groups and major organs. Due to vasoconstriction in the hands and fingers, a natural response is to pump or clench them. It is also a physical manifestation of his plans to strike you.
Trembling: Due to adrenaline, you may notice trembling of the hands or knees. It’s just another indicator that the aggressor is in an agitated state. This is not necessarily a sign of fear, rather it is another physical manifestation of the huge amount of adrenaline coursing through the body, readying it to strike and fight.
Rapid Breathing: Again, this is another indication of agitation and stress. Breaths frequent to distribute oxygen to the large muscles, the heart rate goes up and the body is in a state of excitement.
Avoiding Eye Contact: This could mean a few different things or a combination of them. When a person is trying to mentally process the situation in an agitated state, it is difficult to multitask (hold your attention and think of a plan and psyche himself up). It could also be an attempt to lull you into a sucker punch (i.e. looking over your shoulder so that you turn your face).
Posturing: It is common among many animal species as well as us humans. One of the main indicators is puffing up the chest to make one appear larger and flaring the arms a bit out to the side to make it look like their muscles are pushing their arms out so as to appear more muscular, or lowering the head as if ready to charge.
Bobbing and Rocking: This is similar to trembling and fist clenching above. With the adrenaline getting pumped into their systems, the aggressor may display odd movements. He may bounce up and down, rock back and forth, sway, or bounce his fists up and down. This is an outlet for the dump of extra energy, and indicates a subconscious preparation for action.
Hiding the Face: An aggressor on the verge of attacking may attempt to conceal their stress and excitement by turning their head or hiding their faces. This may also be seen when an aggressor wipes his face, slicks back his hair, scratches his nose, wipes his mouth etc.
Focused Attention: AKA “target glances”. Focused and/or repeated attention on a particular body part such as the chin or groin showing where they will probably strike, or to your gun if you open carry.
Thousand Yard Stare/delayed responses: The person seems to be looking through you/seems to glaze over or "gone". He is mentally shutting down and is ready to go on aggressive physical autopilot. See above “avoiding eye contact”. Attacker is preparing his next move and may be slow to respond to questions. Blink rates may noticeably go way up or way down as their body preps for contact.
Pacing: the adrenaline running through the attacker’s body provides an immense, abnormal burst of energy. This may be manifested by pacing to relieve the body's excited energy.
Scanning/looking over their shoulder/your shoulder: looking for witnesses, their friends/backup, law enforcement, your friends
Mouth/Jaw activity: licking of lips, clenching of the jaw, pursing of the lips, chin jutting forward, lips become pale, talking to themselves out loud or under their breath working themselves up, suddenly setting their jaw- if jaw was relaxed it is suddenly clenched in preparation to better take a counter-strike.
Flaring of nostrils- goes along with rapid breathing- flared nostrils allow for more oxygen intake.
Increase in pulse- can be visible in the neck arteries, sometimes veins in forehead can protrude.
Sweating- profusely, suddenly
Dilation of pupils: eyes widening or squinting noticeably- varies by person.
Excessive animation: unnaturally moving a lot, especially with the hands/arms. This could be to try to trick you into thinking he is just gesticulating when it is really serving to cover an oncoming strike.
Moving the hands up to the face or chest area: he is trying to minimize the distance from his fist to your face
Pointing/poking/pushing: Poking a pointer finger into the chest is an indicator that they are not yet ready for a strike and need to be “pumped” up some more, they are looking for you to say/do something to take them to the next step of being validated in hitting you. Pushing/shoving is also another way a person can make physical contact in an aggressive manner when not yet ready to strike.