It’s important that those who will actually use the manual test it for useability, completeness, and accuracy. With software documentation, you may want to test the manuals at the same time the software is being tested. Try to allow sufficient time for this process to uncover any problems before the manual goes into print. Involving the users in testing furthers their sense of ownership of the manual, and increases the likelihood they will use it when it’s finished.
If the actual users of the manual aren’t available, have some of your co-workers who are typical of real users test it out. Of course, some manuals, such as policy manuals, can’t be tested in this manner.