Check Google's search results.

Go to Google and enter your entire URL into Google's search box. In this example we'll use a
made up name. (www.jkhljkhkjh.com). Let's say this website was in Google's index at one time.
On a side note, I tried www.somerandomdomain.com and www.fakedomainname.com and
both were already taken. I guess some people just don't have enough time on their hands :-)
Anyway, we'll go to Google and enter our entire URL and click "search"
Http://www.jkhljkhkjh.com
Notice that Google says there is no information available for this URL? This means that the
URL is no longer in Google's database.
If you enter a brand new website into Google, you'll always get this message until the website
has been indexed. But, in this case, our website has been banned by Google for some reason.
Another way you can quickly see if your website has been banned is by:
Viewing the Google Toolbar.
Download the Google Toolbar here:
Once it's installed, simply visit your website. If the Google toolbar is completely gray, this
means you have been banned by Google.
Ok, so now we know how to tell if our website has been banned.