A simple Wordpress function for checking whether or not a page is using any custom page template

functions.php

// A simple function for checking if a page is using a custom template
function using_page_template()
{
    $templates = get_page_templates();
    $result    = FALSE;

    foreach ($templates AS $template_name => $template_filename)
    {
        if ( is_page_template($template_filename) )
        {
            $result = TRUE;
            break;
        } 
    }

    return $result;
}

// We don't need to do anything if we're in the admin section
if (!is_admin())
{
    // Sometimes there is an issue with the function not existing, so we define it
    // if we can't find the default Wordpress function (no harm in doing this)
    if ( !function_exists('get_page_templates') )
    {
        function get_page_templates() 
        {
            return array_flip( wp_get_theme()->get_page_templates() );
        }
    }
}