How to check that a player hit a mob from the back?

Check the entity's rotation values and compare to the player's.  Particularly rotationYaw.

In LivingHurtEvent check if source was other entity, get that entity and compare pitch/yaw of entity hurt (remember casting, instanceof) with position of attacker.

 

I'm just adding something more.

@SubscribeEvent
public void hurt(LivingHurtEvent e){
	Entity attacker = e.source.getSourceOfDamage();
	EntityLivingBase living = e.entityLiving;
	if(attacker != null && living != null){
		if((attacker.getRotationYawHead() % 360) + 45 > (living.getRotationYawHead() % 360)){
			System.out.println("Blow from back!");
		}
	}
}

Always works

Attacker yaw: 359

Defender yaw: 0

--> code runs, attacker and defender are only 1 degree off, I wonder why.

Solved. Thanks.

if(attacker != null && living != null){
		double angle = MathHelper.wrapAngleTo180_double(attacker.rotationYaw); 
		double angleTarget = MathHelper.wrapAngleTo180_double(living.getRotationYawHead());
		if(Math.abs(angle - angleTarget) < 22.5D){  
			System.out.println("DUDE");
		}
	}