a script I give to my Unity VR classes to detect when the player is looking at something... put it on anything with a collider, and it'll know if it's being looked at or not

RaycastTargetTrigger.cs

using UnityEngine;
using System.Collections;

using UnityEngine.VR; // we need this line here for InputTracking and VRSettings functions

// USAGE: put this on a thing you want the player to look at!
// this code will enable that thing to know if it is being looked at!
[RequireComponent (typeof(Collider)) ] // this thing needs a collider if we should look at it
public class RaycastTargetTrigger : MonoBehaviour {

	bool amIBeingLookedAt = false;
	public float maximumDistance = 1000f;

	// ADD YOUR CODE AND BEHAVIORS IN THESE FOUR FUNCTIONS BELOW ===============================

	void OnStartLook () { // happen the first frame when I start looking at something
		Debug.Log("user STARTED looking at object " + name);
	}

	void OnStopLook () { // will happen the first frame when I STOP looking at something
		Debug.Log("user STOPPED looking at object " + name);
	}

	void OnNotLooking () { // will constantly happen every frame, while thing is NOT being looked at
		// Debug.Log("user is still looking at object " + name );
	}

	void OnLooking () { // will constantly happen every Update, while thing is being looked at
		// Debug.Log("user is still NOT looking at object " + name );
	}

	// =========================================================================================
	// TRY NOT TO EDIT BELOW HERE ==============================================================
	// =========================================================================================

	// Update is called once per frame
	void Update () {
		// STEP 1: setup a Ray variable before we fire a Raycast
		Vector3 rayOrigin = Camera.main.transform.position;
		Vector3 rayDirection = Camera.main.transform.forward;
		// if we are using VR, we can get more accurate info by reading the tracking info directly
		// (this also fixes a bug in OSX when using Unity's Standard Assets FPSController and MouseLook, where Camera forward is based on mouse)
		if ( VRSettings.loadedDevice != VRDeviceType.None ) {
			// shoot a ray based on the HMD's reported rotation
			rayDirection = InputTracking.GetLocalRotation(VRNode.CenterEye) * Vector3.forward; 
			// do extra correction pass if Main Camera is parented to something
			if ( Camera.main.transform.parent != null ) { 
				rayDirection = Camera.main.transform.parent.TransformDirection( rayDirection ); 
			}
		}
		// actually construct the ray
		Ray ray = new Ray( rayOrigin, rayDirection );
		// visualize all this stuff in Scene View
		Debug.DrawRay( ray.origin, ray.direction * maximumDistance, Color.yellow); 
	
		// STEP 2: setting up a blank var to know where we hit something
		RaycastHit rayHitInfo = new RaycastHit(); 

		// note that a Raycast is an infinitely thin line... which means it only detects things that
		// are in the exact middle of the screen... if you want a "thick Raycast", look up "Physics.Spherecast"

		// STEP 3: actually shoot the raycast now
		if ( Physics.Raycast( ray, out rayHitInfo, maximumDistance ) && rayHitInfo.collider == GetComponent<Collider>() ) {
			// visualize the successful raycast as a red line, using the actual distance from impact
			Debug.DrawRay( ray.origin, ray.direction * rayHitInfo.distance, Color.red ); 
			// visualize the impact point as a small magenta line, pointing based on the surface's curvature
			Debug.DrawRay( rayHitInfo.point, rayHitInfo.normal, Color.magenta ); 
			// ok do logic stuff now...
			if (amIBeingLookedAt == false ) {
				OnStartLook();
				amIBeingLookedAt = true;
			}
			OnLooking();
		} else { // what if the user is NOT looking at this thing?...
			if (amIBeingLookedAt == true ) {
				OnStopLook();
				amIBeingLookedAt = false;
			}
			OnNotLooking();
		}
	}

}