Over at Juicy Studio, they have a tool on testing how readable the content is. You can test any online document by entering the URL into the page. It will return couple of reading level scores, including Gunning Fog, Flesch Reading Ease, and Flesch-Kincaid. Even though they are just algorithms, they can be a good indications how suitable it is for your readers. Here are some background on readability tests:
Readability is the measure of how easy it is to read and comprehend a document. Readability tests were first developed in the 1920s in the United States. They are mathematical formulas, designed to determine the suitability of books for American students at a certain age, or grade level. Automating the process was intended to make it easier for tutors, librarians, and publishers to determine whether a book would be suitable for its intended audience. The formulas are based around the average words to a sentence, and the average syllables used per word. As such, they tend to reward short sentences made up of short words.
Being mathematically based, readability tests are unable to determine the likelihood that the document is comprehensible, interesting, or enjoyable. It's possible to obtain good readability scores with gobbledygook, providing the content contains short sentences made up of monosyllabic words. We'll leave the question as to why the word “monosyllabic” has five syllables for another day. Layout and design are also important factors to the readability of a document that cannot be determined using readability tests. Documents aimed at a higher level may require background knowledge, which cannot be determined by the tests…
Readability Test – [Juicy Studio]