The test is expected to be published and available for purchase next year, offering a chance for parents to intervene at the preschool level. "That's the exciting thing about identifying these three precursors," Wagner adds. "There are successful interventions."

To know whether kids will have trouble reading, the Center for Reading Research tracks development of five skills. Here are the top three they look for.

Phonological awareness. Children who can hear the discrete sounds of their language know how to "break the code" between the spoken and written word. Called phonemes, these are the smallest units of sound that change the meanings of spoken words - changing a "b" to a "p" changes bat to pat, for example. There are 43 phonemes, and stringing a selection of them together can produce any word in the English language. There are nine trillion possible phoneme combinations, but only a small number are used, and they are used in multiple words.

"If you have phonological sensitivity, you can hear which things are similar and which are different," Wagner says. "That pays off because the sound structure is reflected in how we spell the words. Rat and cat have the same middle and final letter. If you can hear where they're the same and where they're different, our method of representing words by alphabet makes a lot of sense. If you can't hear that stuff, then it's completely arbitrary. The learning task is so much harder."

Children typically can't be tested for their ability to distinguish phonemes until about age four. The center's experts discovered that they can determine whether younger children are acquiring this skill by using compound words. Say a compound word to a child and ask him or her to repeat the word without one of the syllables. Like this: "Cowboy. Now you say it back, without saying 'boy.' " This shows whether the child can break a word into smaller components. "If you're good at doing that with compound words, then you'll be good at doing that with phonemes later on," Wagner notes.