Francesco Mapelli has recently launched a new SEO tool called the Supplemental Index Ratio Calculator which allows site owners to check the percentage of pages from their site that are included in the Google Supplemental Index.

What is the Supplemental Index Ratio?

Google has a secondary index containing pages of less importance. This pages are considered supplemental results, and returned in SERPs only if there are no pages from the main index matching the given search term(s). The Supplemental Index Ratio simply gives you the percentage of pages indexed from your website that are in the supplemental index. The lower your Supplemental Index Ratio, the better it is for your website.