unity - Check which object triggered another

Two animated car object both contain a rigid body, a box collider and a script with an OnTriggerEnter event.

I want to check which car started the collision event, by hitting the other car.

Consider two cars; regardless of whether carA hits carB or carB hits carA, both cars contain the same script with the same event. So both cars become the trigger.

How do I identify which car started the collision? I wish to do so without the use of raycasting, as it is too expensive for what I am wanting to do.