There's a fine line in self-defence between reacting to an obvious attack and pre-empting an attack that hasn't quite happened yet.
You're probably sure that you'll do something if an attack is already underway, but you might not be so sure what you should do if the situation isn't so black and white. You've probably also heard your instructor say something like 'if you're aware and know what to look out for, you'll know what to do'.... but do you?
You may know a bit about body language and micro expressions - if you don't, this relates to how the body involuntarily communicates a person's true state of mind or intentions through their posture, movements, and facial expressions (keep in mind body language can be consciously manipulated if one knows what they are doing).
Before an assault occurs, a would-be assailant will typically show warning signs (or, "pre-assault indicators") before or as they are thinking of going in for an attack: Here's a basic list of things to look out for (though these aren't the only things that can happen). *Note: these indicators will usually happen in 'clusters', a few of them happening at the same time, and followed by others*.
Fighting Stance: The aggressor will get into a position that resembles that of a fighting stance (strong foot behind, torso at a 45-degree angle). This indicates their intention to get ready for a fight.
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. This process will cause the jaw muscle to bulge, fists to close and facial muscles to contract. If you pay close attention you may observe the trap muscles rise as everything else becomes constricted as if to prepare for physical contact or assault
Shakiness: Due to adrenaline, you may notice the hands or knees shaking. It’s just another indicator that the aggressor is in an agitated state.
Rapid Shallow Breathing: Again, this is another indication of agitation and stress.
Avoiding Eye Contact: Contrary to popular belief, humans can't multi-task. When someone is obviously avoiding eye contact, looking around from place to place, it is likely because they have something more pressing on their mind - like figuring out the best way for them to make their move. It could also be used as a method of distracting you, so they can get one on you - the classic 'hey look over there' trick.
Posturing: This is mostly an involuntary act and is a way for many species to assert their dominance and intention to attack (keep in mind, intention doesn't always mean follow-through). One of the main indicators is to make the chest appear larger or lower the head as if ready to charge.
Restlessness: This is similar to trembling and fist clenching. It indicates a subconscious intention for action. Just as if you were restless or anxious and you may move your leg up and down while sitting, or start pacing the room, an aggressor may do similar things in response to adrenaline and oxygen being pumped into the body.
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, etc.
Target Glances: The aggressor may indicate their intention to attack by staring at targets such as your face, eyes, nose, or potential weapons-of-defence you may be holding. Additionally, they may be looking for escape routes, witnesses or 'buddies' that they know are around or looking for any objects you're holding that they want to steal from you.
Thousand-yard Stare: This is the opposite of 'avoiding eye contact' but is done for similar reasons. An assailant may seem to blank-out just before they strike. They may stare straight through you like you're not even there. This is probably due to their automatic responses and adrenaline taking over just before they make their move.
Invading Personal Space: Aggressors will use various tactics to get close to you, especially if they are trying to steal something that you have. The most likely way is that they will use seemingly innocuous conversation to give you a false sense of trust towards them, they will move in closer to you, probably displaying a combination of the above signs and then attack. (As a general rule, don't let someone you don't any closer than leg distance).