There's a multitude of ways to generate a 404 page so there really isn't one true answer to this. 404 pages can be generated using the web server default. They can be named anything on the server and the server or site can be configured to serve it up. They can be static HTML pages with a simple name or dynamically generated beasties whose content is generated by database calls.
In very general terms you would have some luck appending
/404.(insert extension that the site runs here) but you would have almost a 100% hit rate just adding a long garbage string to the domain name:
Of course, the latter technique will also get your IP banned by site operators who check their logs on a regular basis...