There is a quick and simple test. Create a new page, one completely free of code of any kind, with the filename test.php (the actual name of the file is immaterial, so long as it follows standard web naming conventions and has a .php suffix). On this page, write a single line of PHP code:
<?php phpInfo(); ?>
Then upload or place the page in the appropriate directory on your server and use your web browser to navigate to the page.
If the server is running PHP, what you will see may surprise you: a huge presentation of data pulled live from the server regarding PHP, Apache, and associated modules. While very simple, this also demonstrates how powerful server-side languages can be: a single function can generate an incredible amount of content.