Even if I’m also not quite sure if this question is related to bbPress or BuddyPress, locating (plugin-) template files might always be the same:
An easy approach might be to call the body_class() in a file which is definitely active (like header.php). The output will look something like this:
class="forum-archive bbpress archive post-type-archive post-type-archive-forum logged-in admin-bar"
If this is not possible one can print the location of the template file via an action. Simply put this snippet at the bottom of your functions.php:
function show_template_name() {
global $template;
print_r($template);
}
add_action('wp_head', 'show_template_name');
Note: This will output the complete path to the current template file on every page! Do not use this on a live site! The output will look something like this:
/Applications/MAMP/htdocs/wordpress/wp-content/themes/twentyfourteen/archive-forum.php
And finally a more sustainable approach might be to use a debug plugin like Query Monitor which will add a Dropdown to the Admin Toolbar like this:
Note: Even if this looks pretty, please consider that it will take a lot of resources and should also not be used on live sites.