[padobson]

This is my answer. How do you know you've learned something? If you're learning how to build something, you know you've learned it when you can build it.

Fiction is a little tougher, but I've found conversations with my wife help me here. If I can make her as interested in the story as I am, then I must be retaining enough of the important points to say I'm absorbing enough of what I'm reading.