How reliable are backups

I regularly plug an external USB hard drive into my macbook to do backups, perhaps every fortnight, i plug the HDD in, click on the clock icon and backup now and leave it to do its thing.

The thing is how can i check if everything is backed up, im looking to wipe the macbook to trade it in and want to be sure all my data is safe.

If you're referring to a Time Machine backup, it's easy to verify if everything has been backed up. Open Time Machine from your Applications folder and then enter Time Machine. Double click on the last date/time you completed a backup and a Finder like dialog should open. You can go through that and check to see if everything is there.