概要

エディタで使用するツール時が変更されると

using UnityEditor;
using UnityEditor.EditorTools;
using UnityEngine;

[InitializeOnLoad]
public static class Example
{
    static Example()
    {
        EditorTools.activeToolChanged  += () => Debug.Log( "changed" );
        EditorTools.activeToolChanging += () => Debug.Log( "changing" );
    }
}

EditorTools.activeToolChanged や EditorTools.activeToolChanging が呼び出されます