Important data has to be backed up in professional environments and even at home where you have important photos, videos, documents, eMail etc which you don't want to loose. There exist a lot of tools on Linux (rsync, rsnapshot ...) to create backups. From time to time you should check whether the backup is identical with the original. It's uggly if you detect the backup is not complete when you actually need the backup.

Following tools exist to compare directories and to find missing directories and files:

diff

diff -rq original/ backup/

rsync

cd original
rsync -rvnc --delete . backup # Without -c ony timestamps and filesizes are compared - no filecontents (much faster). --modify-window=1 should be used  with FAT or NTFS filesystems

hashdeep

cd original
hashdeep -rlc md5 * > ~/checksums.txt
cd backup
hashdeep -ravvl -k ~/checksums.txt *

Links

How to compare the content of two or more directories automatically

File and directory comparison tool?