Find the Flesch-Kincaid reading level by taking the average number of words per sentence and multiplying it by 1.015. Then, multiply the average number of syllables per word by 84.4, and subtract this number from the first result. Subtract the entire figure from 206.835. The closer your result is to 100, the easier the material is to read. Of course, this is just a rough baseline estimate to begin with.
Look for short line widths. Long line widths make it necessary for the reader to turn his entire head, which slows down the entire process and reduces comprehension. This is one reason why newspapers and academic journals are often published in columns.
Look for white space. White space plays a key role in readability -- there needs to be enough to make clear distinctions between words, lines and sections. However, there can't be so much that it's unclear where one unit ends and another begins. This is a gut-level reaction: just read the text and ask yourself whether you noticed the white space or not.
Look at the paragraph length. Long paragraphs are difficult to read because they don't give the reader a "break." Shorter paragraphs improve readability, although longer ones are sometimes necessary depending on the content.