How can I find what nodes are causing 404 errors?
Try a the "Recent log messages" report and apply a filter to show you only the "page not found" logs. You will then see a list of pages not found and you can click on the message to find out more details about it.



If the problem is that a lot of pages change their path then you might want to use the "Path Redirect" module (http://drupal.org/project/path_redirect) if you are using Drupal 6 or the "Redirect" module (http://drupal.org/project/redirect) if you are using Drupal 7 to let search engines know about the new path of the content.

Note: if you have clean URL setup make sure it is working and you are not missing the .htaccess file. It happen to me once when I move a site from dev to production and I didn't copy the .htaccess file.