Test the quality of your documentation by having one or more people unfamiliar with the contents read it and provide feedback. There are two ideal ways to test your documentation:
QA testing is a fantastic way to have your product tested to its limits. This time when a QA tester is working through the product is also a great time to have them run through your documentation. As they set up, use, and modify the product, have them read through the accompanying documentation to make sure it meets their needs.
Early User Testing
If your team has one or more direct users willing to do early testing of your product, what bliss 🙌🏻. When you provide them access to the product, don't give them any form of introduction. Instead, give them your documentation along with the product and see how it works for them. This is a prime way to understand whether you've met the direct needs of your users.
If you don't have a group of users who will do early testing, find one. Direct customer insight in product development is a core tenant of the Agile manifesto. Documentation is a product just as much as software is.
Often testing docs draws out small bits of new information to include, or small adjustments to make. These small changes can play a huge role in reducing support burden in the future.