How To Find Out if Your Website is in Google’s Supplemental Index

...Or "How I Almost Just Had a Heart Attack because I Just Realized My Entire Blog Was in Google's Supplemental Index."

Apparently in WordPress, if you go to Options > General, you can set the "WordPress address (URL)" & the "Blog address (URL)" and this determines how the links are made to your posts throughout your site.

Unfortunately, mine was set to this: http://dennisplucinik.com/blog

This was causing my website to get penalized for duplicate content because the link to my blog from my root domain was to this: http://www.dennisplucinik.com/blog/

I don't remember setting this up without the "www" or if it was like that by default. Either way it caused me to panic. I have however figured out the problem by changing the URL setting from within WordPress as well as implementing a 301 redirect using this following regular expressions in my .htaccess file:

RewriteEngine On
RewriteCond %{HTTP_HOST} !^dennisplucinik.com$ [NC]
RewriteRule ^(.*)$ http://www.dennisplucinik.com/$1 [L,R=301]

I hope that if anyone else is having this problem this post could shed some light on it for you. Here are the resources which I used to help me figure this out:

Supplemental Result in Google - Hell or Help
Get Your Blog Out of Google's Supplemental Result Hell
.htaccess, 301 Redirects & SEO: Guest Post by NotSleepy
View All Your Google Supplemental Index Results
301 Redirects and Domains With and Without WWW

Update:
I'm happy to report that my site is completely out of the Supplemental index (1 week later). Not bad.