Yeah, I like to extend this out and make it generic like this:

If you don't test it, how do you know?

For example, one person told me he can't understand antivirus software and why people buy it, because he never got a virus. I asked him "how do you know you didn't get a virus?". He just looked at me, not saying a word. I hope my point got across though. If you aren't checking, you don't know. Same could be said about hacking these days. You secure your system, that's good, but if you don't have something to detect hackers, you are the same as the guy without antivirus and the guy without tested backups. You just have no idea whether or not you are protected.