Comment by kqr

It would be a fairly short Perl script to read the access logs and curl a HEAD request to all URLs accessed, printing only those with 200 OK responses.

Here's a start hacked together and tested on my phone:

    perl -lnE 'if (/GET ([^ ]+)/ and $p=$1) {
        $s=qx(curl -sI https://BASE_URL/$p | head -n 1);
        unless ($s =~ /200|302/) {
            say $p
        }
    }'