I find it very helpful to be able to monitor my sitemaps in Google Webmaster Tools. It's free and it lets me know how many pages Google has decided to index compared to how many I've submitted from each section of the site.
Here's a picture of it:
Is there a way to add the automatically generated XenForo sitemaps to this view? It seems like there really ought to be, and I could do it with the addon I used to use from @Rigel Kentaurus .
Also, on a rather technical note, while I've noticed that there is a sitemap directory in the internal_data directory, it isn't web accessible. How, then, do the search engines download it? Is it through sitemap.php in the root directory?