As they say on the site:
The Readability Test Tool takes the text on your web page and gives a score for the most used readability indicators.
- Flesch Kincaid Reading Ease
- Flesch Kincaid Grade Level
- Gunning Fog Score
- Coleman Liau Index
- Automated Readability Index (ARI)
You can even get a bookmarklet to drag to your address bar so that you can easily (one click!) get a sense of the page. This is a great tool, well worth checking out.