You can sometimes find files by "backwhacking" a URL, that is, entering partial URLS corresponding to (sub)directories (folders), with a slash at the end. If an Apache server is configured to allow directory listings and a directory does not contain an index.html file (or whatever other files the server recognizes as "index files" e.g. index.php), this will give you a listing of the files in that directory.
However, if the directory contains an index.html or similar file, you will always get that. If the directory listings are disabled, and there is no index.html, you will get an "access denied" message or something similar.