In a nutshell:
Assuming your student has a normal or above IQ, you look for the two signs of trouble: overuse of the right-hemisphere and a poorly operating corpus callosum.

In enormous detail:
First, of course, you have to be sure you are looking at a real reading problem, so you want a reading score of some kind and a rough-and-ready IQ score. If there is a big discrepancy between potential and production, that's an important clue.

Second, you want to test for use of the right brain in reading. This is easily accomplished by having a student read a group of words that have been misspelled, but if pronounced as written, would be real words. Example: fut, brade, blone, or peze. (Foot, braid, blown, or peas, in case your own left side needs jogging!). These "words" are called misspelled homophones. The right hemisphere has terrible trouble with these.

Third, you want to test the state of the corpus callosum. This test is called Tactile Localization, or TL. It is easy to do and, if you can imagine, doesn't cost a penny! How to do TL and the misspelled homophones tests are described in Diagnostic Tests.

Last, you will want a writing sample done from dictation. In an older student, check for bizarre spelling, omission of small words and punctuation, misplaced apostrophes, reversed letters and other odd mistakes. Checkout the before & after writing samples to get an idea of the problem.

A big discrepancy between potential and production is the most important clue.

Here are the sentences I use for students at fifth grade or older:

1. The clam sat on the bottom of the ocean.
2. They rushed into the cottage in the nick of time.
3. We gathered in a circle around the campfire and told ghost stories.
4. Pittsfield has a population of about fifty thousand.
5. A conference was held to determine the future course of action.

If your student is intelligent, reads at about half the level he should, makes mistakes on the misspelled homophones test, writes as though some of the words originated in Outer Mongolia and shows signs of a poorly functioning corpus callosum, you've got him.

What about little kids? How early can dyslexia be diagnosed? Tactile localization is not a reliable indicator of dyslexia on a second or third grader. But his written production is usually a dead giveaway, as you can see from the samples I have culled from little students. But hating school, terrible report cards, and threats to keep back an intelligent child is a sure sign.

Here are the sentences I use for second to fourth graders:

1. Ann must drink her milk.
2. Seven ants had a picnic on my ham sandwich.
3. What are you two doing here?
4. The flowers have many buds on them.
5. Some people come every day to see my father.

If a child in second grade still doesn't recognize short vowel sounds, can't match a letter with its sound, can't identify the individual sounds in a word in the proper order, seems otherwise intelligent, and "writes" like the writing samples in this web site, you may be sure he needs some serious left-hemisphere training.