A物体有CameraB渲染，首先将A物体世界坐标转换为屏幕坐标，再由屏幕坐标的x与Screen.width做比较，如果x<Screen.width则在屏幕内，否则在屏幕外。

Vector3 screen3dPos = CameraB.WorldToScreenPoint(enemy.transform.position);

if (screen3dPos.x > Screen.width)

{

    Debug.Log("out of screen");

}else{

    Debug.Log("in screen");

}