Is there a way to find out which sites are compatible. Trail and error? Or is it up to the site to let members know.
At the moment, just trial and error, unfortunately. But this is going to change.
Remember to add your forums in the "forums.xxx.com" or www.xxx.com/forums" or similar. The valid URL is the one you get when entering from desktop to the forum, removing the file and parameters. Some samples: