Did You Cover Everything???

My friend Cheryl has an article on natural learning. She gives an example like unto matters which all homeschool moms have experienced (but is so good to reminded of by someone who doesn't live in my house!).

In one paragraph she commented on the oft-heard query by non-homeschoolers: "How do you know you've covered everything they need to be taught?" Assuming that the conventional schools actually are the arbiter of "everything that needs to be covered" (which is certainly a debatable point!), then my question is:

"How do you know the students have retained even half of what you've taught them? And what good does it do to cover the material if the students didn't absorb it?"