Check Forms Authentication Status Using Only JavaScript?

The reason i need to do this is because of Facebook Connect - which is another story, so i'll save you the drama for that. =) Anyway, i have this function that runs on window.onload:

function userAuth() {
SomeFunctionWhichGetsFacebookCookes();
if (!loggedInUsingFormsAuth && loggedInViaFacebook) {
window.location.reload(); // refresh page, so i can perform auto-login
}
}

So, i need help in getting the flag "loggedInUsingFormsAuth". I dont care what is in the cookie, just need to know if the current user is authenticated. Why am i doing this? Well, on window load, if the user is logged into Facebook but not on my website (according to the Forms Authentication cookie), i want to reload the page - which allows my ASP.NET website to read the Facebook cookies in the HttpContext and log the user in. I need to do this in JavaScript, because i dont have the Facebook cookies until i call "SomeFunctionWhichGetsFacebookCookies" - which can only be done in JavaScript.

So, how can i work out if the current user is authenticated via JavaScript? Do i have to manually traverse through the cookies, find the one i want, and inspect it? Is this a safe thing to do? Or should i alternatively write out the flag to the client from the server using RegisterClientScript?