You could have a look in the $_SERVER['HTTP_REFERER'] and see what that shows. But not all browsers will actually set this, and it is possible to "fake" and even modify this header.
If the HTTP_REFERER header is actually set from Google, then you'd want these two functions:
[man]parse_url/man and [man]parse_str/man to easily mimick the $_GET array. Something like:
$referrer = $_SERVER['HTTP_REFERER'];
parse_str(parse_url($referrer, PHP_URL_QUERY), $get);
// Just printing to show results
print_r($get);