There is a blog post titled "An Easy Way To Get Banned In Google" that describes exactly how to find out if a site has been banned. The post is at:
Steve Hudgik's Internet Marketing Blog - Internet Results: An Easy Way To Get Banned From Google
It talks about using the Google Site Maps tool available in Google Webmaster Tools. It will specifically identify a site as being banned, if that is the case.
The site discussed as being banned in Google was reinstated to Google. There is a post on that also.