I find the distance between imagetarget and arCamera but its return value more than 100 ..so its return true value ?  

Hi, in Unity I do it like this:

// Update is called once per frame
void Update () {
if (isTracking) {
  Vector3 delta = arCamera.transform.position - this.transform.position;
  float distance = delta.magnitude;
  Debug.Log("Distance to marker: " + distance);
}
}

Put this in a script which is on the ImageTarget

I hope this helps and is correct. I just searched for the part in an older project.

EDIT: Sorry this is only for distance. For Angle I guess you can do something with Dot product and the arCamera.transform.position or arCamera.transform.forward and as second parameter this.transform.position. Look it up. Someone has done it for sure.