SpriteMaskingComponent.cs

#if UNITY_EDITOR
using UnityEditor;
#endif

public class SpriteMaskingComponent : MonoBehaviour
{
// ...
}

#if UNITY_EDITOR
[CustomEditor(typeof(SpriteMaskingComponent))]
public class SpriteMaskingComponentEditor : Editor
{

  public override void OnInspectorGUI()
  {
    SpriteMaskingComponent comp = (SpriteMaskingComponent)target;

    base.OnInspectorGUI();
    if(comp.owner != null)
    {
      string msg = "Instance ID: " + comp.owner.GetInstanceID() + "\n"
                 + "Stencil ID: " + comp.owner.stencilId;

      EditorGUILayout.HelpBox(msg, MessageType.None);
    }
  }

}
#endif