Send a redirect response with the cookie set; when processing the (special) redirected URL test for the cookie - if it's there redirect to normal processing, otherwise redirect to an error state.
Note that this can only tell you the browser permitted the cookie to be set, but not for how long. My FF allows me to force all cookies to "session" mode, unless the site is specifically added to an exception list - such cookies will be discarded when FF shuts down regardless of the server specified expiry. And this is the mode I run FF in always.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…