画面外と中を判定するやつ

InOutTest.cs

using UnityEngine;
using UnityEngine.UI;

public class InOutTest : MonoBehaviour {

    [SerializeField] Transform target;
    [SerializeField] Camera targetCamera;
    [SerializeField] Text text;

    private Rect rect = new Rect(0, 0, 1, 1);

	void Update ()
    {
        var viewport = targetCamera.WorldToViewportPoint(target.position);
        text.text = rect.Contains(viewport) ? "IN" : "OUT";
    }
}