RSS has <wfw:commentRss> for linking comments feed and <comments> for linking page with comments. Atom has <link rel=replies> or just <link rel=alternate type="application/atom+xml"> for comment feeds.
And I'm visiting blog post pages first to check whether they have Disqus comments (which processed separately via Disqus API) and <blog_post_URL>/feed feeds.
And I'm visiting blog post pages first to check whether they have Disqus comments (which processed separately via Disqus API) and <blog_post_URL>/feed feeds.