How to check if one trigger(BoxCollider) is behind the player?

Questions & Answers
1

I use First Person Controller.
In OnTriggerEnter,I will check if the player faces one trigger,I can do this using Vector3(Camera.main.transform.forward,TriggerTransform.forward),but when the player faces one trigger,if player steps backward,another trigger will be triggered,but this trigger is behind the player,
so MY PROBLEM IS How to check if the trigger is just behind the player.

2

How about using raycast, and cast the ray towards the back of the player?