There's a Google Groups thread that is discussing the ways webmasters can tell that their site is penalized by Google.
In case you wanted to cover all your bases, you might want to try the tactics that they suggest you look at:
One person suggests that you will *only* find your site listed when you do a site:domain.com search (rather than searching for the homepae). Another says you won't see it when you do a site:domain.com search either. And another helpful hint:
If aboutus.org outranks you for your domain name, you've got problems and it probably has something to do with links, how you manipulated them, got them, or distributed them.
It's always helpful to check Google Webmaster Tools for any error messages. A user named John adds that Google will notify you of small infractions like hidden text. Not all issues are reported in Webmaster Tools.
Forum discussion continues at Google Groups.