The best way to do this is to read the current forum's permissions for the ANONYMOUS user.
$auth->acl_get_list(ANONYMOUS, array('f_post', 'f_reply'), $forum_id);
This will return an array with the information you want contained within. See docs/auth_api.html in your phpBB download for more information.
A Donut's Blog
"Bach's Prelude (Cello Suite No. 1) is driving Indiana country roads in Autumn" - Ann Kish