Most of the variables used in the forum are specific to their individual templates, and you can find those by viewing the default forum theme.

There are a few global forum variables, however, following is a list:

{charset}
{include:head_extra}
{include:body_extra}
{include:spellcheck_js}
{path:spellcheck_iframe}
{screen_name}
{path:logout}
{path:image_url}
{path:forum_home}
{path:your_control_panel}
{path:your_profile}
{path:login}
{path:register}
{path:memberlist}
{path:forgot}
{path:private_messages}
{path:recent_poster}
{path:advanced_search}
{path:view_new_topics}
{path:view_active_topics}
{path:view_pending_topics}
{path:mark_all_read}
{path:do_search}
{path:smileys}
{path:rss}
{path:atom}
{recent_poster}
{forum_name}
{forum_url}
{page_title}
{module_version}
{forum_build}
{error_message}
{path:theme_css}
{site_url}