Word has the power to not only judge your grammar and spelling, but also your writing's complexity. For example, does your writing meet the comprehension level of someone pursuing a post-grad degree, or someone still learning their shapes and colors?
Word uses lingual tests such as the Flesch Reading Ease test, which calculates a score on a 100-point scale. The higher a number, the easier it is to understand. Office's own documentation suggests you want a score between 60 and 70.
Additionally, Word will run a similar lingual test, the Flesch-Kincaid Grade Level test, which will tell you what US grade level a text is written for; most docs should aim for between 7.0 and 8.0, Microsoft says.
In order to use this feature, you will first have to turn it on by navigating to File > Options > Proofing. Then select Show readability statistics under spelling and grammar. Now when you ask Word to run a spellcheck, it will also generate your document's readability statistics.