Quaternion.Angle  

// Calculates the angle (degrees) between
// the rotation of this transform and target.

var target : Transform;

function Update () {
var angle : float = Quaternion.Angle(transform.rotation, target.rotation);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
public Transform target;
void Update() {
float angle = Quaternion.Angle(transform.rotation, target.rotation);
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

public target as Transform

def Update():
angle as single = Quaternion.Angle(transform.rotation, target.rotation)