Depends what server software you're running and how it's configured. The specific configuration element you want to look for is usually called the "bind address". If it's set to something other than "127.0.0.1" then it will probably accept external connections.

If you know the port number your server is running on, you can also open up command prompt / terminal and check with "netstat -an". Look at the local address column and make sure your web server is listening on 127.0.0.1.