If you have Microsoft Word, put your text into that to check it. You can turn on the readability stats in the Spelling & Grammar preferences. (Note: That option won't show until you select one of the Check Grammar... options.)
Use MS Word Help if you need more details.
If you don't have Word, search the web for another option.
FYI: You'll want a clean copy of the text to check readability. If possible, write the text and check it before you put it into Rise. If it's already in Rise, consider copying and pasting into what text file you'll be using to check readability.