gistfile1.cs

using UnityEngine;

public class Test : MonoBehaviour
{	
	//targetB
	
	private Plane plane;
	public Transform targetA;
	
	void Start ()
	{
		plane = new Plane (transform.up, transform.position);
	}
	
	void Update ()
	{
		plane.SetNormalAndPosition (transform.up, transform.position);
		if (plane.GetSide (target.position)) {
			Debug.Log ("Up");
		} else {
			Debug.Log ("Down");
		}
	}
}