a) crawl your own documentation and find dead links.
b) monitor 404 errors and track down the source (via referer headers, etc.).
c) ask the user on the 404 page what they were hoping for or provide a search menu, record it, and have someone manually review.
d) add a redirect management engine, so you can redirect links that you can't/won't ever fix.
I didn't mean to imply it was trivial, but the concept of crawling your own documentation is quite sound and should just be part of good document maintenance.
There are some plenty good crawlers already. I'm guessing one of them has a 404 report that could hopefully be used to find dead links.
