Sessions and iframes (or frames)?

Hello. I built this site using using different iframes: one for a login, other for a forum, another for a voting poll, etc... I was wondering what is the best way to ensure that you could only post a topic in the forum, or vote if you had already logged in. I'm somewhat familiar with the $_session global variable, but only if the code happens in the same page. How can you check, from another frame, that user is logged? Thanks in advance... Tiago Simões