概要

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

public static class Example
{
    [MenuItem( "Tools/Hoge" )]
    public static void Hoge()
    {
        var isPrefabMode = PrefabStageUtility.GetCurrentPrefabStage() != null;
        Debug.Log( isPrefabMode );
    }
}