Editor下，通过PrefabStage监听打开/关闭Prefab编辑场景事件，通过PrefabStageUtility获取当前正在编辑的Prefab

using UnityEngine;
using UnityEditor;
using UnityEditor.Experimental.SceneManagement;

public class LevelPrefabSceneExtension
{

    [InitializeOnLoadMethod]
    public static void Init()
    {
        //打开Prefab事件
        PrefabStage.prefabStageOpened += OnPrefabStageOpened;
        //关闭Prefab事件
        PrefabStage.prefabStageClosing += OnPrefabStageClosing;
    }

    private static void OnPrefabStageOpened(PrefabStage prefabStage)
    {
        //if(prefabStage.prefabAssetPath.StartsWith("Assets/···"))
        if(prefabStage.prefabAssetPath.EndsWith("Cube.prefab"))
        {
            //在场景视图绘制按钮
            SceneView.duringSceneGui -= DuringSceneGui;
            SceneView.duringSceneGui += DuringSceneGui;
        }
    }
    private static void OnPrefabStageClosing(PrefabStage prefabStage)
    {
        SceneView.duringSceneGui -= DuringSceneGui;
    }

    private static Rect buttonRect = new Rect(5,50,125,20);
    private static void DuringSceneGui(SceneView sceneView)
    {
        Handles.BeginGUI();
        if(GUI.Button(buttonRect,"仅在打开Cube时显示按钮",GUI.skin.box))
        {
            CheckPrefab();
        }
        Handles.EndGUI();
    }

    private static void CheckPrefab()
    {
        //获取当前打开的Prefab
        PrefabStage prefabStage = PrefabStageUtility.GetCurrentPrefabStage();
        if(prefabStage == null)
            return;
        GameObject prefabRoot = prefabStage.prefabContentsRoot;
        if(prefabRoot == null)
            return;

        Debug.Log(prefabRoot);
    }
}