如何使用htaccess跟踪在404上重定向的关键字或链接

I am trying to track the keywords or links that lead to my site 404 page. I am redirecting using htaccess. Is there any way to done this?

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^404 ./404.php