If someone seems to be looking through you instead of at you, If their eyes are darting around or if they are glaring straight into your eyes. Pupils dilated, though can be hard to tell with alcohol/drug affected assailants.
Someone who is ready to attack you typically will have his or her feet set in an aggressive stance, with one foot back and the other foot forward, typically at a 45-degree angle. This position places an attacker’s strong side back to prepare for a strike or quick motion.
Clenched fist or clenching and unclenching fist.
When people become angry and begin losing control of their emotions, this is usually reflected in a snarl or a clenched jaw. You can spot this warning sign a mile off and prepare to deescalate the situation, fight, or run away.
Look for signs of rapid breathing, an increased pulse via the neck or temple and of increased sweating relative to the temperature.
If someone has their weight forward (look at their shoulders) its highly likely they're about to strike