Vector3.Angle  

// prints "close" if the z-axis of this transform looks
// almost towards the target

var target : Transform;
function Update () {
var targetDir = target.position - transform.position;
var forward = transform.forward;
var angle = Vector3.Angle(targetDir, forward);
if (angle < 5.0)
print("close");
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
public Transform target;
void Update() {
Vector3 targetDir = target.position - transform.position;
Vector3 forward = transform.forward;
float angle = Vector3.Angle(targetDir, forward);
if (angle < 5.0F)
print("close");

}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

public target as Transform

def Update():
targetDir as Vector3 = (target.position - transform.position)
forward as Vector3 = transform.forward
angle as single = Vector3.Angle(targetDir, forward)
if angle < 5.0F:
print('close')