The worst time to find out your backup hasn't ran in several months is when you need it :(
Unfortunately that is usually when you find out if it does or does not work.
No matter what backup software you use, they will all generate a report after the job runs (which tells you if successful and any errors that occurred).
(At the least you need to check these every time after backup runs, make it habit around when changing the tapes).
If possible it is even better every now and then to move a few important files to a temporary location, and restoring them from the backup. Once restored open them and see if they are intact.
The reason the log isn't always the best indication of success is if you remember back in the day, after you recorded shows over and over on a VCR Tape the quality kept getting worse, static, lines, etc....
It still recorded the show but these would be considered file errors if in a backup.
So this is just a heads up, if you can't be sure your backups are working, have someone come out and test your backups. Data recovery can run into several thousand dollars to get your data back.
They have several online backup services now, just make sure the amount of data you need to backup is included in the plan you can afford, and remember you are trusting them to maintain your data for you.
See you next time!