lucy24 - 11:18 pm on Mar 20, 2012 (gmt 0)
Use a crawler like xenu or screaming frog to find internal and outgoing link issues, and fix them.
Unfortunately a lot of your links will have to be hand-checked. It's an ongoing time investment. Don't know about xenu, but w3c's linkchecker scrupulously obeys robots.txt so it always comes back with a string of "I wasn't allowed to check this link".
Know what riles me? When I find a bad link and decide that as long as I'm there I'll check the other links on the same page.* Then I write it all up, possibly with some extras like "The site is still there but they've changed their URL" and send to the clearly identified contact person... and never get so much as a form letter back. Grr.
* When I'm facing an essential household chore or spell-checking an e-book or equivalent, so procrastination is of the essence.