Fritz shared his thoughts on writing a while back (I just saw this today though), so I thought I'd post my perspective as well.
My biggest problem with writing is that I can often write extensively on a subject, even while I'm still in the stages of learning and synthesizing information. The impact of this is that I've found myself in the situation of cranking out written works to meet deadlines that really could have used more cycles for the inclusion of better information and more insight on the technology. This is especially a problem when working with periodicals, because the deadlines are tight since most magazines have to be published monthly. I've written one or two articles in the past that definitely could have used another week or two of work. When they were published, I looked back and thought "I'm not happy with how this turned out." So when I started my book on SQL Server 2005, I told myself I would not allow the deadlines to drive me to that kind of behavior. So far, so good. However, I'm still struggling with the process of writing on a new subject that people are still learning about. I'm still learning about the product, elements of the product have changed as I've been writing over the past year, and that process will continue through the end of this year. It is an interesting process, yet it feels risky from an intellectual standpoint. I've tried not to rush the process, but even so I'm still learning as I write. It can cause you to doubt your work.
One thing that I've found that helps me to gauge my own level of understanding on a technical subject (or any subject for that matter) is to give a talk or lead a group discussion on the topic with a specific agenda. I find that if I can talk about the subject comfortably and present the material to a small audience before writing about it that I can easily gauge my level of understanding and comfort with the topic. I've given a couple of talks in the past where the material was still too fresh and I didn't have the command of it that I wanted to have, and that can be a terribly disappointing experience. You don't forget it quickly. However, by going through that kind of exercise I can also make sure that something that I need to write can be verbalized in a clear, concise, and appropriately descriptive and informative manner.