Hook for post and page load

You can use the wp hook and check the global $wp_query object or any conditional.

add_action( 'wp', 'wpse69369_special_thingy' );
function wpse69369_special_thingy()
{
    if (
        'special_cpt' === get_post_type()
        AND is_singular()
    )
        return print "Yo World!";

    return printf(
        '<p>Nothing to see here! Check the object!<br /></p><pre>%s</pre>',
        var_export( $GLOBALS['wp_query'], true )
    );
}

See: wp in codex.wordpress.org and wp in developer.wordpress.org