https://google.com/webmasters/sitemaps/siteoverview Gets you to the webmaster tools, sign up a site. Then go to Statistics and then Page analysis It takes some time after you sign up for Google to display anything here. But when it does it will show the Most common words In your site's content and In external links to your site. This is a great tool to be sure the overall keywords on your site are what you want them to be.
You'll get some power from the main site if you add the link to the subdomain into your nav. Then it would be wise to gain other external links.