I am currently working on some documentation that requires certain readability best practices. Wilie there are a number of instruments to measure this (including the one built into Word), I personally like the Fry Readability Graph. This counts the number of syllables, words, and sentences in 3 100-word passages. To help with this process, I just found a simple way to count this online, at the Fry Readability Program. If you ever need to measure readability, I recommend this simple website to assist with the process.