When a website's main page cannot be opened as in this example, is it possible to find out the full address? In this example, I tried
/index.html without success.
In the case of the above URL, you're seeing the Apache "test page", which is displayed when you start the daemon without either placing files within your web root, or customizing your config file. It's because the default DirectoryIndex doesn't find a matching file, which would normally be the main page of a URL ending in a slash.
The above file is actually just an "ErrorDocument" rewrite, that says if there is no content the server should display the file under /error/noindex.html; so in the above case you can see the actual file under http://www.cetmiolive.com/error/noindex.html to verify it's HTML.
To normally find the type of engine rendering a page on a website, I usually use the same method as you (try index.htm, index.html, index.php, Default.asp, default.cfm, etc.) to see if I can determine the source.
That does depend on a default installation though - if you change your handler (for example, have html files actually be processed by the PHP engine) then you can mask the engine driving the website - but still determine what the file name for the directory index is.
Now, in the case above, when rewrite rules or ErrorDocument functionality is used, it's anyone's game to find out. Those mask the URL or change the way the server interprets URL requests, making it extremely difficult to locate the file.
In this particular case, asking Google for all content on that domain comes up with a blank, in other cases searching with
site:www.example.com in Google could quite easily come up with the main, or most popular, pages.