Integrating WP login to other functionality

I am trying to integrate the wodpress userbase into a customer forum I have developed. Instead of the forum having its own user DB i would like users to be able to login on either the forum or the blog and be able to cross post etc without having to login seperately.

I have tried looking through the codex but I am fairly noob to php and the code base is a bit too much for me to handle.

In the forum I check that someone is logged in to make a post.

if((isset($_SESSION['userid']) && ($userpermtopost == '1'))
{
//echo out html and script for which ever page
}
else
{
echo 'You do not have access to post to this page
}

So how do check if the user is logged in to WP and what are the session vars that are used to identify which user etc