You need to test them once or twice a year, to make sure they work as expected. We’ve seen backups fail/die in “secret”, only to be discovered when they are needed, at which point its too late to do anything.
Once such example, is a customer of a popular cloud service provider, who lost the production web server and when they tried to restore from backup, the found out that there was no backup because this cloud provider on supports ext4 as a file system for all their services including backups.
Another time, a Canadian hosting provider run for 3 years with broken backup systems and nobody noticed until they were needed, at that point the latest usable backup was 3 years old… oooppss.
So no, don’t trust them, always test your backups once or twice a year to make sure they are there and working 
