you ask google. type in your board url into google and see what you get.
type in some keyword phrasesthat you know are on your board and see what you get.
sitemaps are not much help for a dynamic site like phpbb that the content changes all the time.
sitemaps are usually only helpful on sites that stay static most of the time.
the permissions for the bots group are usually the best as default when you install phpbb. that is what has been used for years with the best results.
you may want to add more bots to the group as time goes on and you see what bots are hitting your site. the whole purpose of the bots group is so you can control what they can access.