如何确定app_offline.htm是否发出404或503错误

I have set app_offline.htm to my root directory and it works nicely to shut the site down and only show that page. Knowing for some frameworks (ASP.Net 2), this file will yield error 404 instead of 503, I want to check what code does my site return (for SEO purpose). However, when I enter a file url, instead of return me any error, it always goes back/redirects to the app_offline page automatically. How could I check to see my app pool is returning 404 or 503 now?I have set app_offline.htm to my root directory