How to check parameter before form loads

add_filter("gform_pre_render_8", "services_check_serviceid_pre_form");

function services_check_serviceid_pre_form($form){

	//we need to check with the whmcs database for a valid service id as we only want to allow access to the form for clients who have have a valid and paid invoice

	//check if serviceid parameter exists and if not, redirect
	if(empty($_GET["serviceid"])){

		echo 'Sorry but there was an issue with your request';
		echo '<br><br>';
		echo 'Please contact <a href="support@marbellaguide.com">support@marbellaguide.com</a> if you continue to receive this error';

	} else {

		$whmcsservicesid = $_GET["serviceid"];
// some stuff here to verify the serviceid

	//if everything is okay, display the form
	echo 'Your Service Reference is: ' .$whmcsservicesid;
	return $form;

	}

}