Any way to detect a detach?

I have a Post model with many-to-many Tag relationships, and while detaching a tag from a post, I want to check if that tag is connected to any other post and remove it if it's not. Is there a way to do it? Here's what I want to happen in pseudo-code :

// App/Models/Post.php

protected static function boot(){
   static::detaching(function(Post $post, Tag $tag){
      if(!$tag->has('posts')){
         $tag->delete();
      }
   });
}