Finding number of bowling pins knocked over

I'm making a Unity 3D bowling game but haven't figured out how to tally up the number of pins knocked after the ball has hit.

I have tried noticing when the pins rotation has changed but it doesn't seem to work.

I am destroying the ball and that is how the level ends so and instantiates the next level.

Below is the code I added to the pin. Do I need to have an on-collision event for this script to be called. Currently my console just prints "Pin is Standing" the whole time.

public class Pin : MonoBehaviour {

public int score;

public void Update()
{
    TallyScore();
}

public void TallyScore()
{

    //checking if the pin is standing up (if it's not, the pin's rotation will be not 0)
    if (transform.rotation.x < 1 && transform.rotation.z < 1)
    {
        Debug.Log("Pin is standing");
    }
    else
    {
        //pin is knocked over
        score++;
        Debug.Log(score);
    }
}

public int getScore()
{
    return score;
}

}

Add collider to middle or on top of your pin prefab. Then check OnCollissionTrigger with ground collider and add it to global score controller by other object.