Easy...
Write this script:
<?php
phpinfo();
?>
Save it in your web document directory using an extension that you told Apache to look for with PHP (.php, .phtml, whatever), and if you get a nice report on your screen, you have a working installation.
If you get an error message, or if your script is just repeated back to you as plain text, you've got a problem with the install or you need to let Apache know in its configuration files that the file extension you used is a PHP script.