hi, How to check if a file exists on a site?

There's no guaranteed way to find it, since files can be hidden from search engines. But in search engine you can use site:sitename to focus your search on just that site. i.e.:

site:mirc.com keyword

If you're adept at scripting, use sockets to connect to a website and communicate with it using the HTTP protocol. Although, I think, some 404 pages return 200 OK codes, so there's that to think about; but if you're controlling the web server, shouldn't be much of an issue.