There is no one test for dyslexia. A variety of tests are given and from the results of several tests it can be concluded that someone is dyslexic if they present multiple signs of dyslexia. One of the major signs is that there is a discrepancy between a person’s current academic achievement and their ability to learn. To learn more about the “discrepancy model” click here. However, the discrepancy model should not be used in isolation because there are cases where there is almost no discrepancy and that should not rule out a learning disability.
Here are two websites that provide information about tests for dyslexia and why you might choose each test. If your child is being evaluated at school for dyslexia you will want to confirm that the evaluation covers all the areas of deficit for a dyslexic.