How to know for sure if FastCGI is being used to run php scripts

php_sapi_name() or PHP_SAPI, both work.

http://php.net/manual/en/function.php-sapi-name.php

<?php
    if (php_sapi_name() != 'cgi-fcgi'){
        echo 'I knew fastcgi wasn\'t working.';
    }else{
        echo 'Holy crap, something worked for once.';
    }

Here's a simple test:

1. Create a phpinfo.php file with

 <?php phpinfo(); ?> 

inside;

2. Request http://yoursite.com/phpinfo.php/foobar?foo=bar

3. Check the output of phpinfo and look for _SERVER["REQUEST_URI"].

If this variable is missing, then CGI is used. If the variable is present and correctly set to /phpinfo.php/foobar?foo=bar, then either ISAPI or FastCGI is used. Look near the top of the output for Server API; it should be set to either ISAPI (which means ISAPI is being used) or CGI/FastCGI (which means FastCGI is being used, since we already ruled out CGI).

You can use (on centos) apachectl -M, you'll show what modules are enabled:

apachectl -M:

file_cache_module (shared)
mem_cache_module (shared)
version_module (shared)
fastcgi_module (shared)