WordPress.org

Can I restrict access to only certain categories/posts/pages with this plugin?

No, you cannot. This plugin will make the entire weblog and all the Pages, if you have any, protected against un-registered users.

How can I tell if it's working?

After you activate the plugin, log out, and try to access your weblog. You should not be able to access it. Also, once you log in, all pages should be accessible to you in a perfectly normal fashion :)