The easiest way to verify file checksums is to use "rsync -c". Rsync will usually skip checksum verification of files if both the modification time and size match on the source and destination, the "-c" option tells it to always compute a checksum.

Sometimes I also use the following "one" liner, if it's local.

    diff <(sort <(cd /path/to/source; \
                  find . -type f -print0 | \
                  xargs -0 sha1sum)) \
         <(sort <(cd /path/to/destination; \
                  find . -type f -print0 | \
                  xargs -0 sha1sum))

Another nice rsync tip is that if you aren't using -a, you should at least use -t (preserve modification time). This will make a second rsync faster as it can skip files if the modification time and size match.