"Sit down and listen, children! I have something to read to you. No, now, please. Turn that wii off. Stop hitting your brother. Come on, boys. It wont take long, I promise!"
My long suffering children are my test subjects. I read my stories aloud, and watch for tell tale signs of boredom. The usual subtle cues - glazed eyes, yawns, pleads to let them go now, please, so they can do their homework!
One child hangs on to my every word. Then again, he loves reading. One has the attention span of a gnat and interrupts with irrelevant questions. He's not exactly a bookworm. The youngest cuddles up and enjoys the word flow, but is still of an age where he really needs illustrations when he's been read to. He's below the age range of my work, but loves the inimacy of story time.
I never ask whether or not they liked it, but I do stop and check comprehension, in case my indirect descriptions are too vague, or whether my "show" could benefit from a little bit of "tell" (hey, there's good telling and bad telling apparently).
I hear that editors will take the words "My children really enjoyed my work and tell me it should be published" in a query as such a strong endorsement that their fingers itch to flick it into the "no" pile. My children's responses guide me in terms of concept development and difficulty when I'm developing the story. Ok, so I think their taste is impeccable when they appear to enjoy it, but comprehension is more pertinent.
My main measure of success is whether a story stays with them.
If they are asked to do some creative writing in class, and if the story produced resembles something that I've written (particularly if it had been some time since I'd read it), then I air-punch in joy. The story has meant something to them - I haven't wasted my time writing after all.