How do you audit your Excel models?

I am talking about the more advanced Excel models. Like 10+ sheets with multiple sources and maybe been around for a few years. How do you know if the model is doing what it should?

In my experience as a controller for almost 20 years I simply don't audit the model unless I notice that the result is wrong. But I mean... with thoussands of connections and one little mistake can have huge consequences. I feel like there should be a way to see if everything is correct.