[Update: use this supplemental ratio calculator. Google is selfish and greedy with their data, and broke ALL of the below listed methods because they wanted to make it hard for you to figur out what pages of your site they don't care for. ]
A person by the nickname DigitalAngle left the following tip in a recent comment
If you want to view ONLY your supplemental results you can use this command site:www.yoursite.com *** -sljktf
Why Are Supplemental Results Important?
Pages that are in the supplemental index are placed there because they are trusted less. Since they are crawled less frequently and have less resources diverted toward them, it makes sense that Google does not typically rank these pages as high as pages in the regular search index.
Just how cache date can be used to view the relative health of a page or site, the percent of the site stuck in supplemental results and the types of pages stuck in supplemental results can tell you a lot about information architecture related issues and link equity related issues.
Calculate Your Supplemental Index Ratio:
To get your percentage of supplemental results you would divide your number of supplemental results by your total results count
site:www.yoursite.com *** -sljktf
site:www.yoursite.com
What Does My Supplemental Ratio Mean?
The size of the supplemental index and the pages included in it change as the web grows and Google changes their crawling priorities. It is a moving target, but one that still gives you a clue to the current relative health of your site.
If none of your pages are supplemental then likely you have good information architecture, and can put up many more profitable pages for your given link equity. If some of your pages are supplemental that might be fine as long as those are pages that duplicate other content and/or are generally of lower importance. If many of your key pages are supplemental you may need to look at improving your internal site architecture and/or marketing your site to improve your link equity.
Comparing the size of your site and your supplemental ratio to similar sites in your industry may give you a good grasp on the upside potential of fixing common information architecture related issues on your site, what sites are wasting significant potential, and how much more competitive your marketplace may get if competitors fix their sites.