We recommend the fast and reliable tool at https://readability-score.com. It’s free for pasting in passages that you want to quickly check. It even tests websites. And for a small fee you can use it to test PDFs and bulk Word files.
There is a readability tool built into Microsoft Word. We find its performance in measuring reading level is not always reliable. However it is useful for checking average sentence length and the amount of passive voice in your sentences (keep this under 10%). To use the tool, you must click “Check grammar and spelling” and “Check readability statistics” in Word’s Proofing Options.
Editing software
You can also buy editing software as an add-on to your regular word processing program. Editing software gives you continuous feedback on your reading level and guidance on your style as you draft.
We recommend Stylewriter by Editor Software in the U.K. We have used it and believe it provides good value to writers who have already had some clear language training and who want to keep getting better. (Full disclosure: We get a commission for recommending this product.) Click here for a free trail.
Testing your writing with typical readers
Nothing beats testing your writing with a sample of your intended readers. Clear Language and Design offers a full range of advice on field or usability testing. In the online world, this is called user experience (UX).
For a good overview of user experience testing techniques and when to use them, visit the website of the Nielsen Norman Group.