If your sessions are important to the site as they sound like they are, and you are storing them in cookies, I hope you are encoding the session ID before you are writing it to the cookie...
Depending on what else you are storing in your cookie you might need something like this:
if( issset( $_COOKIE[$name] ) )
$cookievars = $_COOKIE[$name];
if( $_GET['usersession'] == $cookievars['usersession'] )
// We are in the clear, do your thing...
// They don't match, don't do your thing...