As usual it’s most reliable to dump the resulting SQL query and see:
SELECT wp_posts.ID
FROM wp_posts
WHERE 1=1
AND (((wp_posts.post_title LIKE '%keyword%')
OR (wp_posts.post_content LIKE '%keyword%')))
AND wp_posts.post_type="post"
AND ((wp_posts.post_status="publish"))
ORDER BY wp_posts.post_date DESC LIMIT 0,5
The only two things native search is considering are title and content.