You may find that some parts are not being read, perhaps because people are looking in the wrong place for them. You may see that people often follow a path and backtrack. If you can guess what they were looking for, you can make the clues around the link more helpful. Make it clear whether your will accept criticism or suggestions from your readers, and how they should send it. Ask people to solve problems using the document, and report on their success.
Testing takes time.
The decision of how much testing you do is based on the quality of the document you wish to provide. You are balancing your reader's time and effort against yours. If many people will read your work, a little of your time will save a lot of theirs. If however you are documenting some obscure part of a system in which no one other than yourself is likely to be interested, or if you feel that your readers are lucky to have anything available at all, there is no point wasting time testing it. In the event of someone needing the information, they might have to go to some extra trouble to follow several links to find what they want, and then to understand what you have written.