How to track unauthenticated users?

I want to show or hide an email-subscription form depending on whether or not a user has subscribed already.

There’s no authentication.

Should I use cookies?

There’re 3 types of them: conn.cookies, conn.req_cookies and coon.resp_cookies

Which should I use?

I’d personally just the the *_session calls (which uses cookies and session data behind by default).