How can I tell if my character controller is in contact with a wall?

This is a duplicate question, however you would go attach a script to your character and add the following function:

// in js:
function OnCollisionEnter (other : Collision)
{
   Debug.Log("I collided!");
   if (other.gameObject.name == "Wall") {
      Debug.Log("Collided with wall!");
   }
}

// in c#:
void OnCollisionEnter (Collision other)
{
   Debug.Log("I collided!");
   if (other.gameObject.name == "Wall") {
      Debug.Log("Collided with wall!");
   }
}

if you need the information every frame use:

// in C#:
void OnCollisionStay (Collision other)
{
    Debug.Log("I'm colliding!");
    if (other.gameObject.name == "Wall") {
       Debug.Log("Colliding with wall!");
    }
}

hope it helps