Either use wp-load.php (performance taxing) or do the following:
// Capture 'init' event in a plugin placed in /wp-content/mu-plugins/
// This will keep the shared cookie fresh for each load.
$cookie_server = $_SERVER['SERVER_NAME'];
// To work an all subdomains uncomment:
// $cookie_server = strchr($_SERVER['SERVER_NAME'], '.');
// Now check if current user is an Admin and do this: Signal Admin presence by
// setting up a special value cookie that you can detect in your other script.
// Prepare a salt and a hash here caculated from $salt, User-Agent and Remote IP
$special_salt = 'setup a string here others will not know';
$special_hash = md5($_SERVER['HTTP_USER_AGENT'].$_SERVER['REMOTE_ADDR'].$special_salt);
// If the user is not an admin remove the special cookie (if exists)
setcookie('crosscript_auth', null, time() - 24 * 3600, '/', $cookie_server, is_ssl(), true);
// If the user is an admin add the special cookie with the $special_hash value
setcookie('crosscript_auth', $special_hash, strtotime('+1 week'), '/', $cookie_server, is_ssl(), true);
// Now, in your other script, use the $special_salt and $special_hash from here
// to compare to the $_COOKIE['crosscript_auth'], if available.
// That will tell you if an Admin is logged in
}); // PHP 5.3 Closure, just change to named function for 5.2
Just read the comments in the code. I tried to describe the entire logic behind it there.
It's pretty safe and the special Cookie is bound to the IP/User-Agent. With a proper salt you should not have problems unless a very 1337 hacker targets you :) Also it's a start for you to tweak on.
PS: For any other clarifications, don't hesitate to ask.