WordPress : Determine if a query variable is set.

wp-has_query_var

<?php

/**
 * Check if the current variable is set, and is not NULL, in the WP_Query class.
 *
 * @see    WP_Query::$query_vars
 * @uses   $wp_query
 *
 * @param  string $var  The variable key to be checked.
 * @return bool         True if the current variable is set.
 */

function has_query_var( $var ) {
	global $wp_query;

	return isset( $wp_query->query_vars[ $var ] );
}