How can I find all of the forces that come on a certain box?

I have a hitbox on a character, and I want to be able to do X if the box receives a high impact force. Where should I begin with this?

Try this, and tweak the certainMagnitude variable to suit you

void OnCollisionEnter(Collision collision){
	if (collider.rigidbody != null){
			if (collider.rigidbody.velocity.magnitude > certainMagnitude){